What does the term "Compounded RCDs mean?

[ban-all-sheds]

What it would seem really matters is how the car is earthed or not earthed. I had same problem with a boat, the only safe way seemed to be using an isolating transformer, however the battery charger on the boat already had an isolation transformer built in. So for us simple method was to use a class II battery charger and not import the earth. 230 volt was from the inverter even when on shore supply.

Quite a big difference between the two types of vehicles, I would have thought, particularly with a steel-hulled boat.

The boat has an electrical connection to earth. The car doesn't unless you attach one.