From what I have discovered the neutral and ground buss bars in the trailer power center are not bonded to the case. That is, there is no continuity between them.
Since there is no actual ground to earth on the trailer the power circuit is treated like a sub panel. The ground used is the ground wire in the power cable back to the source.
Think of it like an electrical accessory. When you plug in an electrical device like a table lamp or toaster &tc, you don't run a ground to earth from the device when using it. The ground is in the power cord.
The trailer is an accessory plugged in to the pedestal.
edit: I should have read this first. The answer is in the article.
https://www.rvtravel.com/hot-skin-vo...oubleshooting/
This is also why when I use my generator for power I use a bonded plug.
(Unless you are going to run an earth ground to the ground connection on the generator, which nobody ever does)