You have what's called a 'HOT SKIN'. That means that there is a 120V hot lead that is grounding out to the trailer, causing the hot lead to feed thru the skin. When you touch the metal skin of the trailer, you complete the circuit to ground and the electricity flows thru you and you get a shock.
This can also be caused by a 12VDC positive lead hitting the trailer skin, or a misplaced wire somewhere. If you have a voltmeter, put the positive lead on the skin and the negative lead to a known good ground. If none are close by, you can bang a pipe into the ground next to the trailer and use that as a ground for the test.If you get a reading, you will know that voltage is flowing thru the trailer skin.
Try unplugging the trailer and see if you get it from just battery power. If not, then it is on the 120VAC side. I would double check your extension cord and the outlet it is plugged into. I would also try a different outlet to see if it happens there also. Also check your receptical on the trailer for cross wiring. If you have a circuit polarity tester (the little plug-ins with the 3 lights on them) it might tell you the problem immediately.
It might be a simple thing, but also might require a lot of digging to find it. Good luck!