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:
Here is what they look like when taken out: