[twindman]

My first montana had one 12V battery. I was a newby and camped in the mountains with no power. I sat inside reading with lights on most of the afternoon and turned on the furnace around 7 pm, I think. The outside temp was probably 60 around 6 pm and got down to around 35-40 I think. By around 5 AM my battery died.
I had an inverter to power my CPAP and the inverter started chirping about low battery power. So I got up and put on pants and went outside and hooked up the truck and ran it for 30 minutes or longer. I got the battery charged enough to finish sleeping with my CPAP.
After that I did calculations. I think the furnace uses 9-10 amps when running. It seemed to run 3/4 of the time with the temps we had so that means it used around 7 AH from 6 pm to 5 AM -- 77 AH. Which, I think, is close to what a single 12V battery has.