12Volt is battery and should only be in the ON position when you are driving and the trailer is connected to the tow vehicle. That sucker will drain a set of series 27 deep cycle batteries, overnight in a parking lot. Been there, and done that
and will remember to turn that switch to off when parked, in the future.
If not power pedestal to connect to, the propane works, seems to take a while to cool off, but from personal experience, when on high cold, it will start to freeze water in the fridge. And it uses minimal amount of propane.