One big problem with RV dash air conditioners is the fact that most could be getting heat from the engine thru the heater valve in the blend air box.
This is not too hard to check. Locate the 2 heater hoses that are at the blend air box. With the engine warmed up and the a/c on these should be ambient temperature. If not the control valve that is in the heater hose is either bad or if cable operated out of adjustment.
If no in line valve then it is possible the door in the blend air box is not closing.
If your a/c control is the Ford type with the vacuum lines coming out of it make sure it functioning correctly. The lines can leak or be pulled loose or the control itself may be leaking vacuum. If the GM style that has a cable to the in line water valve make sure it moves enough to shut off the flow of hot water.
I put a couple of in line valves in our old MH, a 1987 Itasca. The current owner, my neighbor put a manual shut off in the line because the in line valve is bad again.
On our current '99 Winnie with Ford chassis I had to use a hand held vacuum pump and check the vacuum lines in the dash, firewall, on the engine, and by the right front tire for leaks. I found many and also had to replace the dash control valve.
On some units you can put a temperature gauge in the evaporator and disconnect the blower to see if the evaporator is getting cold with no air flow over it. Should get down to 35 degrees to 50 degrees, depending on the thermostat. Colder than 35 is not good as this will cause the evap to ice up.
Warmer than 50 and it will not cool good with the fan on.
Air intrusion thru the firewall, etc. can be plugged up and does help a lot in how well the a/c cools.