Only thing i can think of now then is one of those surge protectors you might be using... Not familiar with any of those in the circuits... Have never used one before...
I'm guessing if you plug the circuit tester inside the trailer 120VAC receptacle they must be reading a open ground perhaps...
Doing what you have done would have solved the problem if it was my more simple OFF-ROAD POPUP trailer haha...
My shore power cable ties directly into my Converter 120VAC Power Distribution panel where all of the HOT, NEUTRAL, and GROUND connections are at...
Roy Ken