Our experience has been with three group 27 AGM's one in front of the wheel well. We have 120 watts of solar, a built-in propane generator and a 700 watt inverter, the convertor/ charger is a 55 watt device . We have never had dead batteries, we run satellite TV, a laptop, cell phones, lights and radio, and the water pump. The fridge on propane uses minimum DC power.
We have never lost battery power, we are off the grid frequently, I monitor a digital volt meter, when it reads 12.2 volts I start the generator or start the diesel truck motor. Often we run the generator in the morning to make coffee, toast and use the electric fry pan, we continue charging until we see 13.3 volts steady, usually two hours total running time . Two type 31 AGM batteries would serve the purpose. I don't see a need for 6 volt batteries if one fails the system is kaput.