Heater vent ducting. Winnebago Adventurer

I had a 2000 Winnebago adventurer with the same exact "problem". One of the registers had a vent that would close off, bed room as I recall. Winnebago's position was that the vent they installed could be closed but to close another was too much for the furnace and to not close any of the others. Never could heat the front of the coach. If the duct is straight through the coach take the rear register off and stick a light into the duct. Then take the front one off and stick a mirror into it. If the duct is clear you should be able to see the light. Otherwise use a small fan to push heat from the front to the back and make sure your dash heat controls are off so that cool air is not blowing into the front of the motor home.