Bka, you bring up some great points.
We made room for two group 31 batteries on the inside of the camper and planned to have more batteries under the flatbed (both for space reasons and to try and keep the CG of the whole system low. Talking with some guys on a solar forum, it seems you do not want to tie in a whole bunch of batteries in parallel. They actually recommend going with a higher voltage system and batteries as low as 2V per cell (but these are 2V 900ah batteries) to have all the batteries in series.
I can't do the 2V per cell thing and still have a separate battery system in the camper. When the camper is not on the truck I still want it powered. Even in a 12V system I don't have room for 6 batteries inside the camper.
If I go with a 24V system, I can have two group 31 AGM 100ah batteries inside the camper connected in series. Under the camper I could have four 6V golf cart size 220ah batteries connected in series. Instead of connecting the different types of batteries as one bank (and the resulting problems with voltage drops and unequal draw because of long interconnects between the banks), I am thinking I could install a bank switch, either manual or automatic that lets me pull 24V from either bank when the other is low. The charging would be more complicated, but I am pretty sure I could work it out nicely.
A 24V system would mean I would get the 4000 watt 24V input Magnum inverter which is actually similar price to the 3000 watt 12V version but is more efficient. The Nova Kool fridge can also run on 12V or 24V. 24V charging from the MPPT controller on the solar system would mean half the amperage and a lower cost charger. Wiring I^2R losses would be 1/4 that of a 12V system.
To generate 12V for things that must have 12V (furnace fan, water heater, some accessories) I would use a 24V to 12V DC-DC converter capable of at least 20 or 30 amps. I think this would be a lot better than trying to tap off of a single battery.
You bring up a good point about the size of the panel. Perhaps it would be smart to go with a smaller frame and maybe a few more panels instead of trying to get the cheapest price per watt in the large frame residential panels. I am flying a bit blind there as far as knowing wind loads and stresses.
