50 ~ volts in door/doorframe, intermittently

Regarding the shore power cord and it's connections, there are no signs of any burnt marks/loose pins on the male end, and the I was with the electrician when he opened up the junction box that the shore power cord connects in, he pulled on everything and had a good look inside and didn't see anything out of the ordinary. The one and only time that I was at/in my RV when it happened, I was in there with a polarity tester plugged in to one of my GFI outlets, and a multimeter grounded outside with the other lead stuck into my door frame. I was just chit chatting with my neighbor turning things off and on and discussing the possibilities for about an hour and all of the sudden I look at the polarity tester and it was reading "open ground", then I looked at the mulitmeter and it was showing 50v in the doorframe. I began turning breakers off and on and trying to isolate the problem, but then we unplugged the RV from shore power and plugged it back in to see what would happen and it went back to normal, no volts in the doorframe, no "open ground" reading on the polarity tester. I'm in there now doing the same thing in hopes that it will happen again. There was no district thing that I turned on or off that seemed to cause it to happen that I could tell.