That's the classic symptom of a loss of vacuum. The flaps are controlled by vacuum. If it's a gas powered RV, the vacuum comes from the engine. If it's a diesel, they don't produce vacuum so they have a small vacuum pump on the front bulkhead under the cowl. It may be an electric pump or an air-powered pump from the air system. Default to defrost is a safety feature, in a case of failure, at least the windshield is kept clear.
See if the vacuum is working, then trace through the vacuum hoses to find the leak or break.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI