Camping interior Solar power charging systems
Right, I'm glad someone caught Kev's error about adding the amp-hours.
Put in series, add voltages together, amp-hours are the same.
Put in parallel, add amp-hours together, voltage is the same.
Them are nice batteries, though. An update on the classic T-220, with 20 extra Ah. Trojan is a top-shelf brand, very good construction and service life. Two of those could be nestled in some storage area easily and you would have a fairly huge cabin battery. When they get even moderately discharged, like only 30-40%, they would accept as much current as your 90A alternator could spare ( the 90A alternator will make 75A at best, and vehicle baseloads will eat up 20-30A most of the time ), so make sure you upgrade the wiring from the alternator to the starter pole because it could be under high load for an hour or two, if you use the batteries as hard as you plan to. That wiring from the factory is already a little under spec, but in the vehicle as-built it hardly ever carries more than half the alternator output for more than a few minutes so they get away with it. VW almost always underspecs their wire sizes, their electrical design department have always been cheap-ass bastards and besides they really didn't plan on these things being in service for 20 years and then some.
But holy crap, you all pay a lot for stuff over there. 132 Pounds at today's rate is almost $200. I can buy new L16 6V with 375Ah for $225 apiece, and no tax because of solar incentives in our state.
Smaller PV panels have a higher price-per-watt but that 43w PV panel is sickeningly overpriced at almost $10/watt even with the charge controller added in, and it would barely make a dent in your power usage if you have the fridge running much. I would shop for a PV panel alone, don't waste time looking at PV "kits", and for that money you should be able to get into at least a 70w panel even with your VAT and all. The going rate here for panels in that size range is not more than $5.25/watt before taxes (which vary a lot from state to state). That's about 3.5 Pounds. For your plans, you would really do best with a panel over 100w, and at that size over here we wouldn't be paying more than $5/watt (actually less than $4.50 is more like it).
You're far better off buying a bigger panel and a small charge controller separately. A 100w panel would put out around 6A and a charge controller with that capacity shouldn't cost more than about 30 Pounds. If you're actually using the battery much day-to-day with the fridge running at all, you could dispense with the charge controller completely so long as you're willing to disconnect it when you get home, but having a charge controller does let you forget about managing charge, and with a bidirectional battery combiner it would keep both batteries topped off when your rig is parked, so long as it isn't parked in the garage.
The others here could tell you how many amps the fridge uses, and the duty cycle, so you end up knowing the Ah/day requirement for that load alone. Take that figure, divide by 6 peak charge hours/day, and you'll have the amps you need your panel to produce. Multiply that by 17 (no, not 12, around 16-17 is actual panel voltage for standard test conditions) and you'll have an idea of the watt rating a panel would need to have to keep the fridge running day-on-day. That is with the panel in a fixed position facing south all day long; if you move it from time to time during the day to track the sun you might be able to get as much as 10 hours of good peak charge per day, but I wouldn't plan the system around that because the human-intervention factor is the first thing to fall by the wayside.
Nice thing about planning this type of system is that you always have the engine alternator to back you up, and driving from place-to-place for a couple hours or more will top off your batteries. But if you want to boondock for days on end and not need to run the engine, then the simple math above is enough to get you pretty close in sizing the system.
(By the way, I've lived on PV power, house and workshop, for 15 years, and I've designed and built most of the PV systems around my off-grid neighborhood as well as some large commercial systems, so you can take that for what it's worth when considering my advice).
Calculating solar panel size