There are many things that draw power, even when you think they are off. A stereo uses power for preset station memory and a clock. The engine computer consumes power. LP gas detectors, LED thermostats, remote locks, etc. Often overlooked is the TV antenna amplifier, perhaps a compartment or closet light, etc.
If not connected to shore power or solar panels on the roof, it's best to turn battery isolation switches to off. Even that sometimes isn't 100%. I have knife switches mounted on the terminals of my house batteries, I've learned the BIS don't disconnect them 100% in storage. With the knife switches, I've left the RV for months at a time with no battery discharge.