If the heater hose going into the mixer box is getting hot after running for a reasonable time, and the out hose on the other side of the unit is not hot, the valve is not switching.
If it is Evans unit, this is what I would do. Remove the part of the mixer box that the hoses go into and out of. I think there are 3-screws. Forget about the printed circuit part that remains on the overall unit. Nothing to do there. Take the hoses off of the unit, clamp them off to save draining the coolant fluid. Take the unit apart and there is a rotating shaft that acts like a ball-valve. Take that out and soak in white vinegar for about 1/2-hour and clean off the mineral crud that has most likely accumulated and frozen the shaft and is restricting the movement. Lubricate the shaft and reinstall. Check the movement of the shaft by selecting the hot to cold position on the dash temperature selector.
I have had to replace the dash temp selector, a Ford part number, as the selector was not commanding a change to the outside unit that is mentioned above. Another fix, if the above does not work is to get a complete new unit, the part with the valves and the printed circuit. I replaced mine about 2 years ago. The supplier of the original equipment had been changed and the pig-tails were changed so an adapter cable was required. Now, this I did not understand, there were two adapter cables. One was about 8-10 inches long and plugged into he existing wiring and that was it. Way cheaper than the other one that was offered. The other one was a complete wiring bundle that ran to the dash and wired into the dash controls. Very expensive. Not sure why the difference when the short one was easy and quick to fix the heater/ac mixer.
Hope this helps. $3500, I couldn't drink enough beer to make that sound like a good idea.
Don and Marilynn
'04 Newmar Dutch Star 4010
Spartan Chassis/Cummins ISL 400