I'd bet you have a few crushed or bent ducts underneath. From posts on here about heating and cooling, folks efforts to improve upon it, and the fact that trailers are poorly made, a good inspection and overhaul of the ductwork would be high on my work list, if I had that situation. There is also no guarantee that the ductwork is laid out correctly. A mfg might try to overcome Winter's effect on tanks and subfloor comfort by pushing a large percentage of available forced air into the underbelly.
The removal and replacement of any underbelly material has been covered on the Forum. A search will turn up some threads about it. IIRC, Forum member LarryJM did a really good job on his underbelly area.
