Absolutely every time.
1- turn off the circuit breakers to the 30 amp outlet and the 20 amp GFCI outlet.
2- plug in the polarity combo into the 30 amp outlet and turn on the 30 amp breaker to the outlet. If it checks out OK turn off the breaker, if not DO NOT PLUG IN THE TRAILER. NOTIFY THE CAMPGROUND AND MOVE TO ANOTHER SITE OR HAVE THEM REPAIR THE OUTLET. IF IT IS REPAIRED CHECK IT AGAIN AND ONLY PLUG IN IF IT PASSES THE TEST.
3- now plug the polarity tester into the GFCI outlet and turn on the 20 amp breaker. If the polarity tests OK push in the button on the polarity tester to test the operation of the GFCI outlet, it better trip and shut off. If it does reset it and remove the tester. IF IT FAILS THE TEST DO NOT USE THE OUTLET.
4- now with the 30 amp breaker off plug in the power cord from the trailer and then turn on the breaker.
AS A LICENSED ELECTRICIAN THIS IS THE PROCEDURE I RECOMMEND.
On edit: I strongly recommend a surge protector be used at all times.
Started Out With Nothing,
Still Have Most Of It Left!