I had a problem with the dash heat control and after doing some research and testing I was able to resolve the problem. Want to share it with others whom may have the same symptoms. I could not control the temp. of the dash heat (all or nothing).
The system on my Airstream (WH chassis) has a potentiometer on the dash temp. control, which in turn controls a valve controling the flow of water through the heater core.
For a troubleshooting guide and parts go to www.evanstempcon.com
I hope this will help others with heat comtrol problems.