Hi Rick,
Although I'm not familiar with your particular rig, vacuum operated vent controls are pretty common. The dash selector switches the vacuum to different vacuum motors (actuators) to open or close vent panels. It sounds pretty definite that you have a vacuum leak, which means you essentially don't have power to run the vent doors.
You'll need to get into the control panel for the heater and locate the vacuum leak. It could be a split vacuum line or even one that's just worked its way loose and only needs to be plugged back in.
Take a look at those vacuum lines and let us know what you find.