Actually, the suburban NT 30 is about the size for my 26' trailer. It has the option of closing off the 2 round side ducts, and then running the heat down the bottom with an adapter plate. That then shoots down the length of the trailer to the bathroom where it pops up again in the bathroom as a floor register with damper.
The return is up front. So the heat should travel from the back of the trailer up to the front and circulate.
What I wanted to do was pull a small amount off of that duct and insert it into the insulation wrap. (think of blowing into a plastic bag) That way it would warm the tank.
I've called around though, and it sounds like the furnace heat is a bit too hot to come into contact with the plastic tank. There would be a temperature drop, as it travels through the duct, but I don't want to melt the side of an empty holding tank. (They are thermoplastics after all).
I don't know how much cold weather camping I'll be doing. The tanks are 1/4" ABS, with bubble wrap, and then the belly pan. There may be some heat leaks from the furnace into the floor, but I want to avoid cracking the tank because of frozen water inside. I can install 12v
tank heaters, but that would be a drain on the battery if I'm boon docking. (not to mention the cost)
I just wanted to run a couple lines off the furnace duct to warm the tank a little.