The proper differential for a roof mounted RV air conditioner is 18-22 degrees between the intake air temp and the cooled exhaust temp. You might get a couple of thermometers with one at the intake vent and the other where the cool air exits the A/C.
If it does not fall into the 18-22 degree range, you might have dirty or clogged condenser/evaporator coils or clogges intake filters. If your unit has leaked some of the R-22 refrigerant, there is no way provided by the manufacturer to replace it.
If that IS the situation, the entire unit is generally replaced unless you have an Armstrong unit, which has process valves for refrigerant evacuation and replacement. Newer units do not. Most RV repair shops however are not licensed (EPA class I or Universal license required) or equipped to do this type of work. You will need a domestic A/C tech for that.
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