All quite true and this issue has been dealt with, if you are using a generator or an inverter in a boat and or caravan for 240v power supply as a stand alone power source this has no path to earth you so your RCD wont work. If you are using this set up you need to have one of these installed and the RCD thrown in the bin,
https://www.rvdsafe.com.au/product/r...ge-device-rvd/
I had one of these installed into my caravan many years ago once I realised there could be an issue when using the generator, it should be made mandatory that all manufacturers of caravans and boats install these form the start.
In layman's terms it all comes down to how many three pined appliances you have plugged in being run by the stand alone power source, many say anymore than one three pin appliance and you are exposed. In the world of caravanning the generator being plugged into the vans inlet is the first three pinned connection so you really don't want anymore after that. The exposure becomes less when using two pinned appliances but of course the potential is still there.
The chances of having more than one three pinned appliance plugged in whilst you are in a boat is slim but in a caravan it is very likely hence why the RVD becomes your friend. As has been pointed out the chances of electrocution could be considered very remote in these cases but now you know that the possibility exists if you have a boat or caravan that you use a generator or invertor on do yourself and everyone else around a favour and have a RVD installed in place of the RCD.