Many of the chevy based motor homes have a particular brand of HEVAC for the dash. Evans if I am not mistaken and I very well may be mistaken, does not matter however.
They have a problem with the heater control valve, it is a water flow valve and it likes to stick.... Open... When this happens you get the same thing I had pulling away from Church this Noon in my dodge.. I set the AC/Heat control to A/C, set the air flow to Dash Vents, and felt hot air.. (Yup, forgot the temp control it was still set to heat since there was frost on the beast this AM)
With hot water flowing the A/C does not cool, So the Prior owner may have installed a Quarter Turn valve in the heater line.. That is the easy fix.
That is the first thing I'd look for.. then we get to the harder stuff, but check that first.
Next we have control failure.. Since I'm only covering one type of HEVAC, (there are others) I need to know the type of system you have before I can do more.