I haven't read all the comments, but I had the same problem on one of our 5th wheels. You're right about the dealers. They will check it out, they are NOT the factory, they can't help you. One ours one side of the trailer got more airflow from the floor vents than the other. We ripped it apart, adjusted ducts, etc. I have a cfm meter, and contacted the factory with the numbers. Our master and bathroom had close to 0 cfms. They had an excuse for everything. One thing they suggested was running the ac fan to circulate. It was a big fiasco. I ended up putting in a restriction in one of the back lines to force more pressure towards the front. It still was not enough. Long story short...get a space heater and live with it. Sorry.
2015 Georgetown Forest River 351DS MH
2008 Keystone Cougar 293SAB 5th Wheel
2007 Jay Flight 30.5RLS 5th Wheel
2006 Jay Flight 29BHS TT