I have owned 3 winnebagos with the same problem. Winnebago told me to only close the vents that were supplied with adjustable vents. Any more and you will be "trapping" too much heat and damage something. If the ducts are clear of obstacles I was told that's about all you can do. Was told to make sure the front dash heat/acnd controls were set to "max air" or "off" as any other setting provides a direct path from the outside to the inside. My info came from a winnebago design consultant recommended by winnebago. I heated the front with an electric heater.