my guess is you have an open neutral some in the trailer resulting in everything feeding back thru the ground. Your GFI is likely only protecting the washroom area and NOT the whole trailer. The wiring / circuit for it is probably good while somewhere else in the trailer the neutral is open (or shorted to the trailer... unlikely).
PS. what was wrong with the first gfi ... would it not reset / test???
PPS. I take this back... :-) what kind of trailer are we talking about, fiberglass or aluminum... ? You must be in some nice "fields" out there ... :-)