I agree that it is most likely a grounding issue. I would not expect it with only 12v unless you have an inverter in your rig creating 120 vac from the 12v.
Have you just noticed it at a new site/hookup? If so, it could be a ground problem at the power source.
This site shows you how to make a good tester to test the source power before you plug into a new site.
First thing I would do is to take the cover off the inside electrical panel where your circuit breakers are located. Then check each and every connection, all hots, neutrals, and grounds. I found a number of screws that were not tight on mine, especially on the ground bar. A couple were almost ready to fall out. Do this with the AC power cord disconnected to be safe.
Get a little
tester like this one. It will tell you if you have any open grounds to any of the outlets.
Then it gets into more serious testing.