From past experience with a pop-up at Lake Tahoe, I knew the heater was the big draw electrically when dry camping (no hook-ups). When we bought the A-frame, we figured we would need the heater 4-5 hours a night, and wanted to be able to stay up to 4 nights. I ponied up for the dealer to put in dual batteries (dual size 24).
The A-frame heater fan takes about 4 amps according to some sources, so my plan was doable. Not sure why, but had one of the batteries go bad, which took the other one with it. Long story short, I replaced the batteries with 2 Interstate GC-2 6V 232AH golf cart batteries from Costco. Cost was just under $150, including tax.
With 116AH usable, I can easily go the 4 nights of dry camping. We generally set the heater to 60 degrees (used to set it lower), and the composite walls and roof hold the heat pretty well. I've never seen more than 50% run time for the A-frame heater, even on nights in the 30s.
When dry camping in late Fall or early Spring, our water, food, and battery all run low at the same time. Time to move on.
2014 Rockwood A122 A-frame
2008 Hyundai Entourage minivan
camping Colorado and adjacent states one weekend at a time
last trip: Black Hills, Custer SP
next trip: Utah 5 National Parks