[Guy]

Joe, our coach is much older than yours, with Duo Therm original basement air. We had the same issue, and found very dirty evaporator coils were the issue.  ( the ones near where the cool air enters the coach cabin. ) They most likely had never been cleaned because of the interior location of them in the coach. We used  can of non rinsing coil cleaner, (important that it be non rinsing) and were able to spray the cleaner on them with a small nozzle similar to what comes on a can of WD40. Repeated the process several times and all is well again.

Just a thought, but it solved our problem.