The furnace only runs on propane. If you have it disengaged, then the furnace can't run.
If you're just cleaning it at home, a small portable electric heater and an extension cord should be all you need. I recommend the cord over plugging into the trailer's electric as they can be pretty power hungry and might overload the circuit while you have other things turned on in the trailer.
There's no use crying over spilt milk... unless it's on your keyboard.