Bob,
Take a look thru
THIS ALBUM and it might give you some ideas on how to enclose your underbelly and even add some insulation. What I did along with maybe some tank heaters would easily make my trailer a 4 season as far as the underbelly area goes. Some trailers have actual heating duct openings to the underbelly area while others simply run the heating ducts thru the underbelly area. The latter normally uses the thin wall flex duct and is what mine uses and it does a great job in heating the underbelly area when the normal furnace runs.
Larry
