Scott, you're talking about the heater control valve. When you turn on the dash heat, the valve opens up and circulates hot water through the small heater core under your dash. Your dash fan then blows past that heater core, heating the front of the coach. The issue some have, is that heater control valve gets stuck in the OPEN position and allows hot water to circulate all the time, competing with the air conditioning.
You can check it, in the manner mentioned above. It's a metal valve, with the two heater hoses attached, one in and one out. However, I doubt that your year coach is having that issue. It would also compromise your A/C all the time, not just now and then.
Many of the builders place valves next to the driver's side of the engine (hoses running parallel to the engine block). Mine have yellow handles. You should know where these are, in case you ever burst a heater hose that runs the length of the coach. You can close the valves and still drive the coach.
Back to your issue....you don't say if it cools normally at any point and then stops, or starts and stops. My current coach would cool well and then suddenly, poor cooling. A while later, it would cool well again. Mine was a cold control switch under the front hood on the condenser box. It was telling the unit that the condenser was freezing up and shut down the compressor.
If yours is running cold and then warm and cold again, you need to to take it to an expert and let them check the various components.
This is a handy tool to have (Available on Amazon for a few dollars) to stick in your dash vent, nearest you and watch the temps as you drive.