[Creeker]

If you isolate the coach from the battery nothing that runs off 12V will work inside the camper. That includes your lights, water pump, furnace, fridge and water heater. The latter three require 12V to operate their control boards/systems even though they are turned on gas mode.

Two 6V lead acid batteries and a 400 to 800 watt inverter will easily run everything you describe above at a fraction of the cost. I did exactly that for 10 years in our previous camper. Used our Honda Genny for any big load.