Furnace not working on battery

[Montana_1925]

We checked into one of Yellowstone's campgrounds without electricity. Figuring the furnace would work, we sure found out differently. The blower worked great, but no heat. We froze for two nights. We checked the fuses, the battery has 12.5 volts,and the furnace works great on electricity. Is there anything else we should be checking to get this furnace to work on just the battery ? We even installed our new deep cycle battery and it didn't work. Help please.