[luggsey]

Think of a isolated supply like a shaver socket, that's similar to the generator/invertor output.
If you touch one phase and earth there is no fault path.....
You have to touch both lines/phases/lives at the same time to get a shock.
If you earth the supply neutral to ground, like a pme house supply and provide a earth conductor to the metalwork of the caravan then you have a return path for fault current and the RCD would work.
Look up a earth free (is it It?) environment where your protection against shock is lack of earth.

Belatedly I just skipped through the document you linked to, its excellent and describes better then I can why the RCD won't work.