We have possibly the same problem and our previous dealer tried to fix it a few times. When we first looked at the coach, the heater hose (hot air) was folded and crimped with a barlock. We brought it to their attention and the service managers & salesman said they knew about it. They took parts from another coach on the lot and also said they ordered new parts, but it never did blow cold air once the engine did more than idle.
Our new dealer, Robert Crist & Co., is now working with Monaco and Massey's Diesel Repair to get it fixed. If Crist doesn't have the technicians to fix it, we're sure Massey's will. There was a question if it was a chassis or box problem.
Not sure where you are located, but if you need cold air from the dash I could post a photo of our crimped-off hot heater hose.