First check to see if the fan and compressor are running. If not, plug into shore-power and then go inside with a voltage meter plugged into a receptacle. Turn on the air conditioner and wait a couple of minutes to see if the compressor kicks in. The voltage should be 103.5 volts. Next, check to see if the light on the thermostat doesn't light up isn't getting any DC power. Check the fuses or circuit breakers, and if this is the problem, simply replace them.
Another problem that may occur is that the fan runs, but the compressor doesn't. Check to see if the cause is low voltage. The problem could also be poor air flow. This is found if the compressor goes on then stops, and it doesn't cool down the RV. This problem could result from a dirty filter, dirty condenser or a leak in the duct work. If this is the problem you need to clean or change the filter or fix the duct work.
Check the filters if the compressor doesn't run at all. It is best for a qualified mechanic to do this. They might have to test the condenser to see if that is the problem and you need the proper machines to do this. Poor air flow can be caused by a number of things including short cycling, a dirty filter or event evaporator freeze-up. You may need to fix piping, clean the filters or tighten gaskets. Also check to see that the holes for condensation are clean.
