I pushed out all the water in the lines and water heater with air, drained the water tank and used a simple space heater (typically used under a desk). I set it to keep the temp in the trailer at ~ 50 deg F and kept a check on it every couple of weeks.
My electric bill was about 15 buck more per month but was worth it to me. The following spring I had a camper that was completely free of mold and mildew, required nothing other than filling the water tank, purging the lines and water heater of the air and loading up for the first trip of the year.
The heater I use has a wide base to sit on, a tip/trip sensor to shut it down if it ever tipped over. I keep it on the kitchen counter about 10" off the wall. No issues here...
