One must remember that the RV TRAILER frame ground comes from the SHORE POWER CONNECTION at the camp ground pedestal.
We are all at the mercy of the good maintenance at each camp ground we visit.
Used to see folks checking the pedestals before hooking up using good voltmeter and circuit testers. I suspect most of us including myself just plug-in and hope for the best. The inexpensive circuit testers (LOWES-AMAZON) only look at the 120VA 15A/20A standard household receptacles which requires you to use RV ADAPTERS to read the 50A and 30A pedestal connections. Who has the time to drag all the adapters and KILL A WATT meter out before you hookup...

You are hoping to SEE the two LIGHTS ON for the CORRECT bottom reading.

You are hoping to see the SAFE ZONE centered on 120VAC... Anything different than the 110VAC-125VAC SAFE ZONE I will not hook up my trailer.
I have a similar setup like shown here from google that I carry with me for my 30A Shore Power SETUP TEST... This is about the only good use I have found for the small round RV30-15A adapters.

Roy Ken