The green wire (safety) ground in your power cord is there to prevent just that occurance. It drains off any stray voltage that may get onto your trailer frame. That stray voltage could be from a frayed wire, a bad electrical device, induced voltage from a normally operating device, etc. If a direct short to AC hot occurs, it would normally trip a breaker. If it did not trip the associated breaker, it would likely generate more than a low grade shock. So it is probably a collection of static induced voltages produced by normal electrical activity in the trailer. In any event, it should be corrected immediately.
There is probably an open circuit somewhere in the green wire ground link. That could be at the pedestal outlet, in your connector to the pedestal, at the connector to your trailer, or somewhere inside the trailer itself.
A tester like this is very good to keep in your trailer to check if AC connections are correct.
If you really want something extra usefull, here is
a tester you can make fairly easily and cheaply that will test the pedastal before you ever plug in.
This RV Electric web site is a very good place to fully understand the electrical system. Click on "Outlet Testing" to get a lot of info on troubleshooting.