I spent 4 days dry camping last week at Road Atlanta just north of Atlanta. It was in the high 40's at night and I did use my furnace in order to keep me and my grandchildren warm. I have 2 Trojan T-105 six volt batteries. I don't know how often the furnace came on or ran during the night, but we were warm. In the mornings the battery gauge in the 5'er was down one light the first morning only. After that first morning the indicator light stayed on full (we all know how accurate these lights aren't???).
This probably isn't a good example past the first night because I have an on board generator and had to run the generator several hours each day so the grand children could watch Sponge Bob. Obviously the batteries charged back up during the day. If that first night is any gauge, I could have made the entire 4 days including heating at night on one charge, but I'm glad I didn't have to find out. There is always the potential of not having enough charge left to pull in the slides and I have three.
Nothing better than a generator.
2006 GMC 2500HD CCSB 4x4 Duramax/Allison, Titan 52 gallon fuel tank, Prodigy Controller, B&W Companion Hitch
2010 Jayco Designer 35RLTS, Cummins/Onan RV QG 5500 EVAP, Progressive Industries EMS-PT50C, TST Systems 507 TPMS, RV Flex Armor Roof