Welcome to the forum. I see no responses to you so far so thought I would start with a response at least. I donít know your coach dash air control specifics at all, but if it uses vacuum controlled switches to operate air flow control doors, typically a leak or failure in the vacuum lines usually defaults to all windshield defrost vents working, to help ensure windshield clearing as a safety measure. In your case the defrost vents are not working at all you say.
This might suggest the air control door for the upper defrost vents may be jammed in some non windshield vent position. I am attaching a basic dash control diagram for reference (which may not even be applicable to your coach), which seems to show that when controls are in the defrost position, no vacuum is connected to the defrost door control motor normally (left side of diagram). This suggests a jammed or failed air door; maybe from some leaves or other contamination (broken return spring maybe) that is preventing the defrost door from returning to ďnormalĒ open position. Might also check if an upper defrost feeder pipe has come off the control box. Getting into the dash; air box, will likely be ugly though.