That condition has happened to me twice.
Once at a mom and pop campground. The electric receptacle was wired backwards.
Once when I used a friends repaired extension cord that was wired backwards.
I have a light on my 1973 Argosy that tells me that the polarity is wrong. Problem is that I never check it. All of my cables are good and I was the one who wired the receptacle that I use at my house. I usually boondock anyway so it is rarely an issue.
THE BEST WAY TO TELL IF THE POLARITY IS CORRECT IS TO HAVE A POLARITY CHECKER PLUGGED INTO AN AC RECEPTACLE AT ALL TIMES.
THE WORST WAY IS TO STAND BAREFOOT AND TOUCH THE CAMPER.
After the polarity has been checked, you should periodically administer the kiss test ( gaze into her eyes before and after ).