First, test the tester to make sure it works correctly. Then, starting at the power source, test at every junction. Test the house outlet/pedestal outlet. Test at the adapter, if using one. Test at the extension cord if using one. Test at the breaker box, and so on. When you find the first location with no ground connection, you have narrowed down the possibilities immensely.
I had to do that when my RV was new. After about a year, I installed a
Progressive Industries EMS-HW30C, and it would NOT allow power in. It said open ground connection. For a year, I had not known there was no ground connection. I started at the pedestal and tested at every junction. I found that the ground wire had pulled out of the wire nut where the shore cord was tied in to the RV wiring.