Excellent read - Thanks - looks like I can't use a battery in that space w a higher than 95Ah
You *can*, and lots of people *do* but it's not clear from Mercedes whether you *should*.
What if I add a second Aux battery - say under passenger seat? Another 95Ah. An AGM? Can these work in tandem? As far as I can tell - I am looking at pretty low amp hrs burned per day...45+-
Re: your amp-hour calculations, you have done the right thing by calculating your daily consumption, but will you drive every day? Do you want a little bit of padding in the calculations so that you can last, say, two days off the battery? It seems to me that you're worried about this question yourself because you're talking about adding more capacity.
If you add another 95Ah battery, you'd be meeting one part of the Mercedes guidelines, because you'd be using batteries that were the same size and the same chemistry. However, the system would "see" your aux batteries in combination as a 190Ah battery, so you haven't magically avoided the issues. Making sure the battery cables were similar lengths (so they charge/discharge in tandem) would be a real pain.
You also have to drive enough hours each day to actually charge the batteries up. Lead-acid batteries have a long absorb cycle. You could find that you're never fully charging your batteries. If they don't get a full charge every month or so, their lifespan will be reduced.
To run thick enough cables between the driver and passenger seat base, it's likely that you'll need to remove both seats and bases so that you can unclip the top on the plastic cable race cover that runs between them. Or run the cables out from the boot under the driver seat, under the van, past the exhaust pipe, and up in to the passenger seat base. You'll then have to route your cables, create a battery tie-down location, and find a way to vent the battery to the outside world (through the floor). It's not clear that you'll actually gain anything by doing that.
If you're going to that hassle, why not do something like a Sterling battery-to-battery charger and install a proper deep cycle battery of a size that's more appropriate?
Would I be wasting my time hooking up solar panels into this set up?
Into the stock aux battery? Maybe. Maybe not. You could use solar to keep your aux battery charging as your fridge depletes it. There's no reason not to, but it's hassle and expense that you could avoid if you are driving frequently enough to keep the battery topped off.
Again, that battery isn't really designed to be deep cycled much. It's up to you to decide whether the solar is likely to add to the depth of discharge (which it would if you feel confident discharging the battery overnight because the solar will recharge it the next day) or not.
As soon as you go to a larger "house" battery, solar starts to make more sense. The more charging you do from solar, the less the alternator has to make up. I'm sure people will shout at me that the alternator is basically free power, but if making it work harder than its design parameters means it burns out faster, then that's not really free. Also, solar works whenever the sun's out, rather than relying on you driving the vehicle. Last I checked, Mendocino's pretty sunny.