Fred,
It sounds like you have a vacuum line off or a bad vacuum pump. The mode doors in the dash climate control system (AC/heater) are controlled by vacuum just like most of the systems in cars, and since your diesel engine has no vacuum source like the gas engine in your car, the vacuum source to operate the mode doors is a vacuum pump that is located on the firewall.
On your coach it will be a small electric pump about two inches in diameter and six or eight inches in length. The pump can be wired two ways. The most common way is that it will run when the key is turned on until the desired vacuum level is achieved or it can be wired to only run when the system is turned on.
The easy way to check the pump is to first check for loose vacuum lines. If they look OK. Remove the output line from the pump and turn on the key. The pump should have a slight vibration in it if it is running, if not turn on the AC fan. If the pump still does not run, check the voltage at the pump to be certain the it has voltage and ground. If there is voltage and ground to the pump and it does run with the hoses off, replace the pump.
Gerald
