Just like a household furnace, except gas has to be on at the tank/s.
Yes, the RV furnaces run on 12v DC battery power, and they do use quite a bit of power for the fan, a typical RV furnace will draw 8 to 9 amps of power. If you have one 100AH lead acid battery it would run one about 4-5 hrs continuous before your battery should be recharged. If it's a cold night, and your battery is undersized you may get cold overnight......