When I had no dedicated heater, I would use an old fashioned pressure cooker. Filled up with water to the max recommended, I would heat it on the stove, but make sure that no steam escaped. Even after turning off the stove, the thermal mass will keep it pretty toasty for a while.
I was in the Pacific Northwest in an Aliner in Juneuary (not a typo). Water was dripping from the ceiling. I did the above and the trailer dried out in just 3-4 hours. Of course I turned off the stove while sleeping. I kept a window and the overhead vent cracked.
Ted