Might be hard to track, and it might be that your neutral and hot wire are crossed in the plug OR In the receptacle you plug your trailer into.
First of all when trying to address this DO NOT BE THE LINK TO GROUND.
Do not touch the trailer and any ground at the same time EVER until the issue is resolved.
I am going to emphasize that in this condition, touching a ground and the trailer at the same time can be DEADLY.
Get a volt meter, and with the trailer UNPLUGGED turn off ALL trailer circuit breakers. touch one lead to a KNOWN good ground (the ground prong in an electric receptacle at your residence SHOULD be good), and touch the other to the trailer skin.
If you have voltage, either the trailer plug or the receptacle wiring is wired wrong.
If there is no voltage, WITH THE TRAILER UNPLUGGED, flip the breakers on one at a time, and when you are safely out of the trailer plug it in and check the skin for voltage. Continue the process until you find the offending circuit.
There is some danger here, if you are the slightest bit unsure of what you are doing, hire an electrician. Just putting a hand on the trailer in some circumstances can be lethal.
My bet is that the problem is going to be in the plug or receptacle.
1/2 Ton 4WD Truck, 72 Sovereign