I'm not sure becuase of the RV setup, but the problem is a check valve for the vacuum controls. Usually under the hood on the passenger side, this valve allows the system to hold vacuum, when there are conditions where the engine is not producing vacuum the check valve keeps the vacuum resevoir from loosing it's accumilated vacuum. The valve may also be under the dash on the passenger side. If the valve is not the concern, than the airflow door inside the heater box will most likely be stuck...
~Rip
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