Dash Air Blows Only Hot Air

Our 1996 Prevost Vogue XL 40 has OTR Air/Heat as well as driver's air and heat. On a recent trip back home, it was cold so I turned on the main bus heat switch on the dash (which supplies heat to all but the driver's area). I also turned on the dash air and turned the temp control knob all the way clockwise to maximum heat. It wasn't long until we were very comfortable so I turned it back off. A few hours later, after outside temps had warmed considerably, I turned on the dash air (without turning the OTR air compressor on) and turned the temp control knob all the way counter clockwise to cool. This will normally cause room temperature air to blow out the dash air vents, but this time the air was very hot and would not revert to room temperature. So, I turned the OTR air switch on and waited a little while. The air coming out in the main part of the bus was nice and cold but the dash air continued to pump out very hot air. The engine coolant that enters the dash air's heater core is controlled by an electric solenoid valve (located behind the front bumper) so I checked it's coil's resistance and it tested good (according to the specs) at 49.9 ohms.

Since getting back home, I have not yet had the time to leave the OTR air running in order to test to see if the incoming solenoid voltage is correct (24V). Does anyone know enough about this type of HVAC system to suggest what I should do from here? Thank you in advance for your help! Blake