Not an A/V problem, but a vacuum assisted controller leak.
There is a check valve that will go bad in the line by the vacuum tank.
When there is a leak you will loose the control that keeps the air mixing door set for A/c only and bring in outside air or in case of a total loss, the system will revert back to the default setting of defrost only.
There is a present thread in the Class C here on this and non-dealer fixes.
Ford truck and van forms on the web are full of these complaints and fixes.
On a Class C the dealer wants $900 due to labor.
DYI fix as posted on the Class C forum is under $20.00 and maybe an hour time.
