Yes, it's certainly do-able. Since the heat ducts run in the floor, what the mfrs do is install a heat duct that blows warm air by the holding tanks. Then insulate around the tanks and enclose the bottom with that plastic stuff. Click the photo for more info.
I just got done doing just this to a good friends camper. It was on a Keysetone but the same should apply. It wasnt my idea but he had plenty of beer so I said I would help. Job went pretty well and seemed to work.
It may be hard to describe but Ill do my best.
Make sure that you whole underbelly is clean and in good order. Once this is done it will be tough to access. We also mapped out and diagramed stuff that may need to be acessed later.
Using flexible duct hose tap into the closest heat duct and run to area of tanks.
In the areas around the tanks block off about 6 inches so that the expanding foam that is sprayed onto the underbelly (next step)creates a ductlike space around the tank . Make sure that the flexible duct hose emties into this area and secure in place.
Have the bottom sprayed with something like this. Expanding Foam Insulation, like you are seeing on houses now and then. This was the only step we had to have a proffesional contractor do.
Then seal the bottom up with corrugated plastic like COROPLAST. Comes in 4x8 sheets. I believe we used the 6mm thick stuff.
I believe the plastic tarp like material Burbman mentions is usually placed above the frame during manufacturing, not to the bottom of the frame. If you did this I imagine you would rip the tarp off the coach from the wind turbulence under the trailer.
This was done on my friends camper because he uses the trailer for 4 weeks of hunting. Gets cold up in Maine.
Any questions shoot me a PM I would be happy to answer anything you may need help with.
* This post was
edited 10/01/07 01:43pm by John & Christine *