GFCI and an Open Ground

Here's a couple of diagrams that show how the 2nd coil detects a grounded neutral current flow. If a path exists between neutral & ground after the GFCI, it completes a loop along with the normal neutral/ground connection at the main service panel (or sometimes the CG pedestal) and enough current flows to trip the GFCI. AFAIK, the grounded neutral coil has been required for at least a couple of decades. A load doesn't need to be connected to detect a grounded neutral and can sometimes cause confusion when trying to diagnose why a GFCI has tripped in an RV.