Well, the easy part to this is with the free electricity and being available you should use the electric and not anything powered by propane which runs out and gets expensive to keep refilling.
The concern with electric is how much electric? 15 amps, 20 amps, 30 amps, 50 amps? 15 or 20 amps usually means you run only one electric heater, then where do you place it so that it can run for long periods and not damage what's close to it, but doesn't cause a circuit breaker to set. With 30 amps you can run more heaters and distribute them throughout the rig for more even heating. 50 amps? Then rent out the rig as an apartment and make some money.
I think if it were me, I would minimize heaters that have fans, or those cheap filament heaters. I thought the radiator heaters might not reflect enough heat and aren't as efficient. Sorry I have only what not to use, hopefully someone will come along with what to use if you can provide how much power will be available. I'm guessing one or two 15 amp where one of the 15 amp can be adapted to the house circuit. Then there's concerns with running extension cords.
Other questions, Is the trailer winterized? If the electricity is plugged in are the converter and batteries still plugged in? This will affect how much electricity you can use for the heaters. How much other stuff is still "live" and can it be disconnected to save amps?