GFCI and an Open Ground

When I looked at how they do this, I discovered that the GFCI simply induces current into the wires which in turn trips the very same current imbalance detector. So while no external current is flowing, there is some inside the device. Regardless, an RV without a ground path will not trip from current imbalance or shorted neutral & ground.

The downstream neutral-ground fault detection does not use the current imbalance detector, but rather a different mechanism. An example can bee seen here: http://www.onsemi.com/pub/Collateral/FAN4147-D.pdf

Diagram on page 2, description starts near the bottom left of page 5. Older designs did use 120 Hz. signal injection, but still used a separate transformer for detection.