Is the Fedders a heat pump like the Cool Kat that is now used on the Roadtrek?
My 2010 Roadtrek with the Cool Kat seems to have the same problem - when set to cooling sometimes it seems to blow cool but not cold air. I finally isolated the problem (sort of) to the thermostat setting. If the thermostat isn't set cold enough to cause the unit to switch from heat mode to cool mode it doesn't blow cold air. If I set the thermostat down real low and turn on the A/C it will kick into cold cooling mode. After that I can set the thermostat to any temp I like and it blows cold air whenever it cycles back on.
It may be that the reversing valve in the unit isn't switching on the selection of A/C or heat but on the temperature difference measured someplace. Setting the temp down real low appears to be enough to get the unit to kick into cooling mode.