An update here after almost six years without the A/C. The van still handles great without it, but on a recent overnight with snow/slush on the roof, we noticed a leak.
Over time, the white surround with for the original A/C developed very small cracks, allowing water to seep in. Unfortunately, my approach to sealing "both sides" of the hole trapped moisture in between both layers of the top. This caused some discoloration (fortunately not rot) of the wooden structure around the hole and some partial delamination of the upper cabinet.
Removing the white surround took a good hour with a sharpened putty knife and a mallet. (One of those multimeister tools would have been useful.) From this exercise, it's pretty clear to me that the silver on our van's top -- original -- is paint, not gelcoat. You can see a mask line around the hole.
I roughed up the area and used 3M 4200 marine adhesive sealant (basically like Sikaflex, but available locally) to bond a sheet of 0.04"
3003 aluminum to the roof. This gauge material is stiff enough to resist flex from snow, but still workable with tin snips and by hand to match the curvature of the roof.
I further secured the panel with eight
special coated stainless self-tapping screws, then gave those screws healthy blobs of sealant. When the sealant fully cures, I will add a duct tape patch to each. (It doesn't have to be pretty up there -- it just has to work.)
Obviously if you have the talent to do what
tneuer did, that's better. But this is working so far and I will report if there is a future failure.
Oh, one more thing: You can do all of this without a big ladder if you aren't broad-shouldered. The bath skylight dome pops off with two little hinge pins (be gentle) and a stepladder will grant you access. It's a bit of a contortion to get both arms up there, but it worked for me. You can also go through the main skylight, but be careful applying weight to the roof.
I will try to post a few pictures soon.
Stay dry,
Ted