Camper electrical problem. Help please

It could also be that your shore power cord, the adaptor, and the extension cord are operating at a high amperage draw with a less-than-good contact in the mix. As the three items are used the adaptor or part of either cord set are deteriorating and causing the ground fault. With the new adaptor, you may have hit a home run.

This is where that meter comes in handy, you can hook it into each cord or adaptor and find if continuity is lacking in a piece of the system.