Beats me, I don't. Except, of course, for the ground wire in the electric service cable. Which is necessary when connected to shore power in case something goes wrong with the electrical system. This provides an attractive path for any current to follow, which is better than through your body.
We have owned RVs since 1972 and have been fulltime for seven years and you have a new one on me? I have no idea why you would worry about grounding it. If you have a proper shore-power connection that will provide a ground for the RV and if not, this electrician has no idea of any reason to want one.