This is typical RV construction quality. In most 5th wheels the bedroom and bathroom is fed off of individual round air ducts.
You will need to access the junction box where all of your duct work splits off from the back of the heater. Make sure they are all connected and have no kinks in them. You might also need to get some aluminum duct tape and tape up all of the connection points.
Also check to make sure the distribution box is connected properly to the back of the heater. Mine was just taped on one side and when the heater came on it would open up and most of the heat went into the basement.
2015 DRV Tradition
2014 GMC Duramax DRW Crew