Dash A/C blows out of defrost when climbing or head wind

If it is Evans, here's their manual. Your coach being a 2000, my money is on a small leak in one of the actuators. A fixable problem once you chase it down. Just one of those problems that takes a lot time to find and cheap to fix once you do. Be so costly to have a shop fix it because of the diagnostic labor involved. It will make you appreciate what the A/C techs have to deal with.

http://www.evanstempcon.com/pdf/ElectricalSystems/Dash_All_Electric_HVAC_System_Service_Manual.pdf