I found this article on RV heaters a few months ago, and with the kind permission of the author, copied it into our Technical Library. The article is quite comprehensive, and cuts through a lot of the advertising hype that generally accompanies discussions on this topic. Because it is lengthy, I have posted it as an attached Word document, rather than putting all the text and pictures directly into this post. If you have any trouble retrieving it, send me a PM.
http://damouth.org/RVStuff/PortHeat.shtml
Bill