2 Battery banks built from 6 volts

You got a lot of good ideas so far, but based on your setup and where you are camping I would stay with your original idea of putting the other two batteries in the truck bed and then tie them into the alternator charging system of your truck. I imagine you use the truck to site see around the area that you camp in so you will get a lot of charging in from your trucks alternator. You will not need to run your furnace with a buddy heater in a small TT so you will save a lot of battery power using it. I dry camp for a week with 2 6v GC batteries and never go below 50% SOC, but have minimal battery requirements. But after 7 days I am out of water, and wife has to do laundry so I have to move anyway. Besides lights and water pump what other requirements, will you have, TV,computer use, watching movies? The 2 batteries in the truck bed would be insurance that you never run out of battery power. How many gals of FW in your TT set up?