[JRTJH]

One point to consider about "heating" the underbelly is that there is more than "just a 2" duct" down there. All of the heat ducting, both the 4" aluminum foil round ducts and the square aluminum ducting to the floor registers is "open" to the underside of the cabin floor. This transitions in most fifth wheelers (and some travel trailers) to a large aluminum duct under the bathroom floor. These "uninsulated" duct systems "leak heat" into the sub-floor and that heat then "leaks" into the underbelly. So, it's not just a 2" duct heating the tanks, but, by design inefficiency, the entire heating system "leaks" into the sub-floor and underbelly.

Certainly not an efficient way to heat, but as much heat is "lost" to the underbelly as radiates up into the cabin from those uninsulated duct runs. Of course the hot air that remains inside the ducts does directly heat the cabin, but much of that heat is lost through those uninsulated duct runs into the subfloor.