I vote for bad ground connection.
1. outlet not properly grounded
2. power cord/extension cord with bad ground wire/pin
3. ground wire came loose at RV. (This happened to me.)
Pick one.
I tracked down my loose ground wire where the shore cord connects to the RV by unplugging the RV from the pedestal. Plugging a known good extension cord into an outlet in the RV, and checking for continuity between the ground pin of the shore cord, and the ground socket of the extension cord. When it showed no continuity, I knew I had to trace the ground circuit from the shore cord to the breaker box.