We had a similar problem in one of our fifth wheels. The main duct (plenum) had become partially disconnected for the furnace. We had heat in the main area but the bathroom and bedroom were cold.When I opened the furnace and found the issue, I was able to reattach the plenum and made a piece of flashing to cover an area that still had a small gap. I also reduced the size of the opening that pushed hot air into the underbelly of the trailer. This takeoff was the same size as the ones for the flex pipe that they used (not the best way to run heating). These two changes made a great difference for heat in the bedroom. Our new trailer has an actual duct that better distributes the heat.
Hope you get it sorted out.
Goody Two Shoes and the Filthy Beast
2008 Chevy Silverado 2500HD
Duramax and Allison 6 speed
2012 Wildcat 282RKX
You can't make things idiot proof, you can only make them idiot resistant.