I do electrical, you have one of a few things that it is--
(In order of what i would say is most likely)
* Since you say this is a modified RV, they very well could have redone your shore power cord, and it could be wired with hot on the wrong side---I feel this is your most likely issue.
* The outlet you're plugging into is backwards.
* Your neutral line is bad, forcing current to now pass through the ground wire, leaving either the ground wire, or you, to ground the trailer
In any case, as Time2Roll mentioned, get an outlet tester and it will tell you
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