Dashboard vent blowing hot air

[Raining Star]

I didn't know if this should go in the Winnebago forum, the Ford chassis forum or the RV systems forum.
My 1999 Winnebago Chieftain 36L is blowing hot air when the dash board a/c heater system is set to vent with the temp dial all the way counterclockwise into the blue area. Even when the a/c is on it seems like there is still warm air mixed in with the cold.
Any idea what's involved in fixing it, something a stumbling home mechanic might try, or is this a specialist item?

[BFlinn181]

Check the linkage to the control valve on the heater hose going into the heater core. I'm not sure how yours operates, mechanical or vacuum operated. Many of us install a ball valve in the heater hose to cut off all heat flow in the summer.