Well, it's in! It was a lot of work, and there's no way I could have done it without the pioneering efforts of Autocamp, Adolphus, Discus, and others (Mike Sisk?) who were brave enough to go first, as well as others (Zach Woods, Theo Engbrink) who did the original research.
Approach
First, the toilet itself. I snagged one for $499 delivered from this eBay shop:
https://www.ebay.com/str/cmeboating. Thetford calls it the "Right" model (the orientation is as if you're sitting on the toilet) but be careful, because I have seen some confusion on various sites. The model delivered was the C402C, which differs slightly from those others have installed -- and that shown in the eBay picture -- in that it only has a "full" indicator, rather than the three levels for flush water (it uses its own reservoir) and waste. I believe the fancier model is the C402X. The one I received does have the cassette with passive venting and a provision to install a vent through the floor.
I had never worked with fiberglass, so I was very nervous about modifying the shower pan. In particular, I was not confident in my ability to fabricate a "dam" at precisely the right spot of the toilet end of the pan, especially given that half the reason of doing this project was because of shower leaks. As a family of four, our shower gets heavy use, sometimes in all four seasons.
With that in mind, I took a different approach, building atop the pan rather than cutting it. As you'll see below, I still had to remove and patch the “half moon” of the NAFTA toilet platform, but did so in the confidence that the area is flat and can mostly be substituted with the material removed.
This process was tedious, and if you count my labor at much more than minimum wage, it makes the
€420 custom pan from Germany look like a bargain, provided you can get it delivered. (The best shipping option for that thing, as far as I can tell, is to fly to Düsseldorf and take a mini-vacation to get one, because Deutsche Post/UPS/FedEx won't ship it, and the freight routes are probably even more expensive. While you're there, stop at any camping shop to get the different drain needed for that pan. You might even get your 19% VAT back if you fill out the right forms.)
Disassembly
Getting everything apart was easy enough. I found I was able to remove the pipe into the black tank by cutting the Sikaflex seal at the top of the tank and working it back and forth with some dish soap and patience. Likewise, I was able to disconnect the outlet by raising the black tank and carefully removing the four 10mm bolts that hold the slide valve in place. Fair warning though: one of these bolts was so corroded from leaks at the slide valve that it snapped. If that happens, you have no choice but to tear everything apart; I was then able to remove the broken bolt with vice grips, but this would not have been possible in situ.
Behind the black tank I found a bit of narrow drain hose not connected to anything. I gave it a tug and it came off in my hand. Any idea what it or the riveted drain tube it attaches to is? The A/C condensate drain is outboard of the toilet wall, so that’s not it. Maybe something from the A/C retrofit done on our van, #121?
Once the toilet, cabinet, and
drain (don't remove its strainer, if you can avoid it) are out of the way, and the perimeter is scraped clear, the shower pan just popped out. I feared a struggle with the adhesive between it and the floor, but time (and leaks?) gave it trivial grip. Fortunately, even despite significant water leaks (on our van, there is corrosion on the brackets supporting the floor, and flaking paint on both ends of the silver trim visible at the rear doors) there was no damage whatsoever to the plywood floor, as it's covered in a waterproof laminate.
Fabrication
I tried to no avail to remove the rear wall of the bath, as I figured it would be much easier to cut the cassette-access hole with that board out of the van, as well as refinishing the silver trip piece at the bottom. I also see that the Euro vans don't use a hole as such, but a large notch instead, which I figured would be easier than trying to cut a perfect rectangle in such a tight space. (See the first photos below of the Euro execution of the cassette access.) But even after removing all the screws (inside and out), the sealant in the rear corners of the bath, the hand towel rail, and cutting the mastic between the rear shampoo shelf and the plywood wall, I couldn't get it to budge. I suspect the walls are bonded together, or there are a couple of invisible, inaccessible brackets. I fished around with a powerful magnet, but couldn't find anything. Save your effort and leave that wall in place!
I was able to cut the hole just fine with some carefully triple-checked measurements and a simple jig saw, from the outside. There was just enough room. I used a fine wood blade, but in retrospect, a metal blade and tape over the cut line might have made a smoother edge. I used a bit of garage door weatherstripping and a staple gun to make it neat.
Incidentally, the sealant in the corners of the bath is apparently made of something unobtainable in the U.S. like
https://www.otto-chemie.de/en/homepage-construction/ottoseal-s-100 in Crokus or Jasmin, depending on which source you read. I used Sikaflex 221 in Colonial White, (not regular white) and found the match acceptable. For the floor, I used the Dow RTV-739 as recommended by Onemanvan. But I'm getting ahead of myself.
The shower pan had a sticker on the bottom suggesting it is made of polystyrene. I have a few bits of scrap left to send out if anyone can do a chemical analysis on it. The drain protrudes below the floor, so you need a thick board with a hole to get it to sit level. From here, I made a couple of observations: the toilet platform is molded so as to be level when there is a 52mm spacer beneath. The plywood platform in the van is only 49mm. As a result the platform is under stress, evident in tiny cracks around the edge. Having these measurements correct was essential to getting a level surface for the cassette toilet.
So, on with it. I measured out a generous platform (I think 28+ cm from the back wall, or about half an inch more than the depth of the cassette) and got at it with an oscillating tool. (After reading Autocamp’s praise for his Fein Multimaster I have a new favorite device. I bought the inferior but much less expensive Harbor Freight clone. You’ll need this to make a dent in the plywood platform anyway; don’t even bother trying a pry bar, because you will get no closer to removing it and will chew up the floor in the process.) With careful work, I kept as much as possible of the half moon intact.
I then found that one of the plywood supports for the black tank fit just perfectly below the platform. I carefully trimmed it down to 52mm wide, then screwed it into place. A couple of scraps of 3mm laminate floor between the platform and pan will fill the aforementioned gap.
I cut a hole for the vent. More on that later. Then, I went about filling the sides, carefully cutting 1/4” plywood plates with a jig saw, gorilla gluing them together into two solid chunks, and bonding them in place with “Liquid Nails.”
I did a test on scrap and found that hot glue adheres well but also comes off without damage. I glued a piece of plywood to the top of the shower pan with a piece of cling wrap over the half-moon hole, then placed the half moon back into place with Bondo. Cling wrap is low-density polyethylene, which like the HDPE yogurt tubs, won’t bond to polyester resins.
After a bit of sanding, I got to try my hand at fiberglass for the underside of the half moon. I used the same stuff as Autocamp: “Bond Coat” polyester laminating resin from Tap Plastics. Two layers of their modified twill cloth went down well enough.
I used some expanding foam clamped down under cling wrap to fill the gap between fiberglass and the plane of the bottom of the pan.
Then Theodoro’s Body Shop of Horrors got to work. It took just more than a quart of Bondo to get everything filled in and curved right. I made a little template to copy the edge profile across the new section. It’s all a bit of a guess because the floor is sloped so all sides aren’t the same height. I checked and double-checked for level.
During the sanding of the top side I found a hairline crack about four inches long on the edge of the pan by the rear wall. How it got there, I have no idea.
Then two more layers of fiberglass, this time on the top side, and with the white pigment from Tap Plastics. Work from the center out, using a disposable brush. Trim it carefully after it gels but before it gets really firm. I found that the gel time was about right — 25 minutes or so — but the time to solid was overnight; longer than I expected. I didn’t sand between coats. I put one last coat of pigmented resin on with Tap’s surface cure styrene/solvent stuff but not sure it made the cure any harder. I sanded the edges for fit but left the rest as-is; it looks a bit rough up close but I fear that sanding it will expose the glass fibers. Any advice now that it’s in place? Experts, please weigh in on how this job should be done! It's not too big a deal now that it's covered in Duragrid again. (I was able to fit pieces together to fill the space again, with the aid of some zip ties that are out of view under the sink.)
Assembly
The toilet attaches to the back wall with four number 6 by 1 1/2” wood screws, not included. There are optional brackets but I don’t think they are needed. Not sure what the Euro version uses, but there’s an extra outside screw on the pictures I have seen. Unless you have a very long drill bit, you won't be able to drill pilot holes using the toilet as a guide. I used the screws to mark the wall, freehanded the drill, and put it together, and it seems to be working.
I spent $16 including postage for the special “vent kit” from the UK. Save your money, as it’s just a flimsy tube and a foam seal. I managed to connect it with some pool vacuum hose to the original black tank hose. Fine for now, but Autocamp’s method is better; should some liquid come out of that vent, it will drain to the gray tank rather than collect in the tube.
For the power flush, I just tapped into the feed for the propane detector. It’s right there and easy. I agree that the hack of the black tank sensor is more elegant.
I still have to finish closing the hole in the floor. There was a lot of rust there, due to the leak. I cleaned it up and hit it with Hammerite, then Owatrol oil between the metal and plywood. A sheet of aluminum is now Sika'd into place. I am hoping to due a "skin graft" of the gray rubber floor somewhere for a plywood patch.
Overall impressions: (+/- vs NAFTA Bravura)
For
+ The power rinse swirl is much better.
+ You can fill the flush tank with RV antifreeze and use it all winter.
+ You can keep the tank clean by back-flushing it every dump.
+ All the water you would have used for toilet flush is bonus, and not out of the main tank.
+ The seat does a much better job of staying up, and I don’t think I will need to install the little magnets that kept the Bravura from falling mid-stream. (That engineering sign-off must have been done by a
Sitzpinkler.)
+ Wet cell is easier to clean without nooks and crannies.
+ You get a ton of new cargo capacity -- enough for a large hard-sided suitcase, several crates of beer, etc.
Toss-up
+/- The capacity of the cassette is about half the original black tank, but you can dump anywhere (without moving the van!) so that’s a wash in my mind, if not a slight advantage for the cassette. The thing is heavy when full but not too bad since you only have to lower it to ground level, not the other way around.
+/- Odors and mess: (All without chemicals.) During use, because the opening of the valve is a separate operation from the flush, there is a hint of odor. For comparison, try flushing the Bravura with the water switched off. Closing the lid eliminates the smell. When dumping, you are closer to the mess (i.e., you can shove the sewer hose down a hole several feet away) but on the other hand, you don’t have to walk through a puddle of someone else's sewer or face backsplash on the ground.
Against
- You lose two little areas in the bathroom on either side of the toilet that are good for flip-flop storage.
- A foot flush is more convenient than the hand valve and button flush.
- When installed on the raised platform, it is exactly the same height as the original Bravura. The seat, however, is much smaller. (See photo below compared to a standard residential toilet.) Men who are particularly well-endowed or have a few extra pounds may need to “throw a line over the bow,” so to speak...
- A clear disadvantage is the lack of a reserve capacity when full. In the NAFTA version, you can literally fill this toilet to the top of the bowl. This isn’t recommended, because you’re relying on a couple of soft seals, but you can do it in the middle of a stormy night. With the cassette, the slide valve comes with you to the dump, and anything in the bowl will leak into the cassette compartment. Heed the “full” light well! You have only 1.5 liters left, which is about two to four good pees.
Supplies
I am happy to give a plug for Tap Plastics here, because their videos were helpful and their products worked well for me. You need almost two quarts of polyester resin, one small bottle of hardener, 2 bottles of pigment, and 3 yards of modified twill cloth. (You will have some left over.)
As usual, I will give updates on this modification and am happy to answer questions here or at Westyfest. Pictures coming soon.
Ted
That is why I can never throw anything away. Not even an expired silicone caulking tube. 
