Improperly wired electrical wiring, from reversed polarity to missing grounds, can charge the metal framing of a trailer. It will not discharge because the rubber tires act as an insulator to ground.
How the charge dissipates is through the touch of a person's hand, creating a complete circuit to ground for the charged frame, entering the hand and exiting through the feet.
Be sure to plug in a trailer to a well-maintained electrical outlet that has a snug fit on the plug. If repairing a personal power cord, note the polarity and where wiring is to be matched so wires are not reversed, causing a charge to build on the metal trailer framing.
A shock can possibly cause heart attack or death. Verify any wiring before applying power.