So it only happens when you are plugged into shore power or have inverter turned on??..... you have a short in the 110v system somewhere and power is going into the trailer frame and skin. Attach a ground wire to skin or frame and drive a ground rod into the dirt until you figure out what is going on. When you touch the trailer you are becoming the ground for the current to travel through since the rig is on rubber tires so insulated. Very dangerous as you could get full shot of 110v which can kill. Best not to use shore power or inverter until you get it figured out and corrected.