Bill & I winter camp, always have. Even in our pop-up (starcraft) we choose to use the ceramic type heaters instead of the heat pump in the AC unit. They just kick butt! Already this winter we've been out in freezing temps and have been as snug as can be inside the TM. We do use two heaters as the rear of the TM is definitely a separate climate zone--much like a two story house. We even power the second heater directly from the pole, and not run it through the TM's grid.
One hint though--a few pieces of pipe wrap cut to length to fit up between the bed pan and the sides of the TM do a great job cutting down that edge-of-the-bed cold draft!
Be sure to tag your draft dodgers with a long-ish piece of ribbon or string so you remember to remove them before you try to slide the beds in.