Not for longterm, but you might experiment with those chemical handwarmer things. They come in several sizes from toe to torso sized, make heat for 10 hours, and don't cost much. I put one by the water pump and one by the pipe that likes to freeze; both are inside the RV, where it is usually only 10 degrees warmer than outside. They worked great on my coldest RV night ever, about 12 degrees outside. I did forget the exterior water drain valve and it froze and burst. Oops.
For longer term, maybe a very small, like 5W, incandescent light bulb?
