Some years ago the standards for wireing changed,,, Back when I was a wee little lad it was fairly common practice for the nuteral and ground wires to be connected together.
This will pop a GCFI 9 times out of nine.
The standard today is different. I'm not absolutely sure what the standard is for the power pole in fact (i'm a bit out of date) but for the trailer it should be seperated all the way to the plug.
This was pointed out to me some years ago... I'd re-wired my trailer for 30 amp service (3 wires all the way back to the plug, nothing common) and was used to using 30 amp outlets.
However at one place I parked the box was 20/30 amp, the other camper had 15/20 amp cord plugged in to the 30 amp via a pigtail adapter (such as you are using but the other way around) because in his rig the nuteral and ground were common and it popped the GCFI's
Now.. personally... I would rather he had fixed his trailer, and not just so I could have had the better outlet (I got by with the 20 just fine that trip, I only need 30 when extending my stay by using electric heat, which I used in the trailer, or electric grill and ac and a few other things all at once (electric hot water heater + AC + Grill + "Burner" = tripped 20 amp breaker, any 3 of them won't trip)
As someone else said, it is a safety issue, I would endevor to have it fixed.
I got one circuit on my new MH (The one I added) where I have to track a gound fault... Tester shows good ground... Trust me. it's wrong