[Richard]

Jon,
You might think about a couple of other factors. The first would be potential leakage of cold air into the front of the coach while driving. In both coaches I have found significant air coming in. The second is the plenums for air distribution need some TLC after a few years. The foam seals inside break down, and I found that as built the plenum didn't work as it should.

I took it out, put it on the bench, and using a vac generator to actuate the motors, I was able to get air going where it was supposed to.