A couple of questions: Has it always done it? Or did it just start doing it?
If it has always done it there may be a reversed wire, or possibly a wire has a fastener through it somewhere along the way. There may be a mis-wired plug.
If it just started doing it, I would suspect a damaged wire, possibly caused by rodent electricians or a fastener through the cable, or the cable rubbed on something exposing a conductor.
There are several ways to test the line to find the fault. What I would do is disconnect the camper from the power, disconnect the first plug downline from the GFCI, cap the wires and plug it back in, if the GFI trips it is in that first run, then continue down the line until you isolate which section is bad. FWIW on my current RV I have 5 down line from the GFI, not my preferred system, I would rather have them on a couple of separate circuits.