:h
You've got a small class C, right?
Assuming the fridge/furnace/stove/water heater are propane-fired, it seems to me that just about ANY (single) decent deep-cycle battery should get you two nights of boondocking. At least! The furnace fan and water pump are your main power consumers- be cagy about pump use especially.
All bets are off if you've got an all-electric rig, of course. Or a big screen TV, microwave, etc.....