Had same problem on my Gulfstream Freightliner.
Evans valve in forward generator compartment was not working. It closes the line bringing hot water from the engine to the heater. If it does not close the line, the AC is running and the heater is effectively running - and you get hot air.
The Evans repair kit changes the number of wires used to hook to your controls on the dash.
The solution I used was to insert a manual valve in the hot water line from the engine to the heater (before the Evans valve.) I now close it in the summer and open it in the winter. All is good.
I will probably install the new Evans Valve before I sell the RV, just so the new owner doesn't have to see a non-factory solution in a first class Gulfstream Class A.
36' 2005 Gulfstream Cresendo, 350 Cat, Freightliner
2002 Jeep TJ toad, +2", Smittybuilt, MT and BFG all around