Start here by watching this informative video on RV electrical systems...
Batteries are the heart of the 12 volt DC system. If the batteries are hooked up and not disconnected by a switch, then something is always going to use DC power from them. Lights, power "brain" modules for the fridge, furnace, water heater, etc., propane leak detectors and maybe smoke detectors, etc.
The largest draw while using the RV is usually the furnace. The blower motor uses 12 volt power from the battery.
If the RV isn't being used, like in storage, locate and use the battery disconnect switch if equipped. Otherwise, manually disconnect the cables from the battery to remove all power draws from the battery.