There is a bunch of stuff that does not work on battery only power. Don't screw around with two 6 volt batteries wired in series. Instead, use two 12 volt deep cycle batteries and wire them in parrallel to each other. Your two batteries in a diesel pickup truck are wired this way.
TV, Stereo, AC, Microwave, Fireplace, need 120AC to work. Don't know about furnace and ceiling fan as I never tried those on just battery..
Fridge, lights, landing and support legs, slides, awining, and water heater all will work with just the battery.