Denise, (Templeton, MA)
This type testing typically is beyond the scope of the average RVer. It’s probably best to make an appointment at your local RV service center. If it only happens while running off the generator power, the problem could be solely in the rear A/C unit itself. Typically it’s the thermal overload device which fails in this instance. The overload is attached directly to the compressor on the A/C and can be tested for continuity with a Volt-Ohm meter. If the compressor is overworked, due to low refrigerant, etc., then the overload device may trip. It acts like a circuit breaker of sorts and may be fatigued enough to warrant replacement. I’m guessing it will take further investigation to determine the exact cause of your symptoms, but I would recommend you check that overload device first.