Usually this is an indication that the 120VAC INPUT level from the camp ground pedestal is too low for your air conditioner compressor.
I use one of these plug-in 120VAC METERS in a good spot where I can glance at it to check the 120VAC LINE VOLTAGE before turning on the Air Conditioner. If the LINE VOLTAGE is not within the MARKED GREEN SAFE ZONE then I will not turn on the Air conditioner. Pick one up from local RV store or AMAZON.
Continuing to power-up the air conditioner if all you hear is a loud hum may damage your compressor motor.
The RV Air Conditioner Compressors are usually not equipped with FREON TEST PORTS for A/C repairman to use.