If you are planning on doing a lot cold weather camping, may I suggest installing an inexpensive, but way more efficient, hydronic (hot water) heat system.
Google the following. This article got me on the right path.
Simple RV Hydronic Heating System - Ibiblio
BTW, I install industrial and commercial HVAC systems for a living (have done so for over 30 years). And even though I'm a "duct work" guy, I tell people, "unless you absolutely need duct work for central air, don't install forced air gas heat... install hot water heat." IMO, all rv's should be built this way, and hopefully they will soon be.
http://www.youtube.com/watch?v=cZ5i8FzU4UE