Walt, don't be so quick to judge. While it is true that a gravity furnace is not very efficient, and the surface temperature gets quite high, they also require no electricity, and provide enough heat for three season camping. They are
light, (ish) simple, quiet and fit nicely over the wheel well. Typically they use space that otherwise would be wasted.
Here is what they look like:
http://www.fiberglassrv.com/forums/f...tml#post174423
Here is what they look like when taken out:
http://www.fiberglassrv.com/forums/f...wer-54226.html