It could also be the receptacle you are plugging into having a broken GROUND WIRE. This is the only way the trailer frame gets it's EARTH GROUND through the GREEN wire coming from the shore power connection.
Pick up one of the very expensive circuit tested described above. Great to have around anyway. Just plug this into the receptacle using the adapters and what lights up.
This could be a very dangerous situation. Different folks reacts differently to small electrical shock. My fellow worker back in my working days would actually touch a 120V or 240V hot wire with his finger and tell you what the voltage was - TRUE STORY... A very TUFF OLD SC man...
Roy Ken