My first thought is that it's a compromise. It would be ideal if an RV's frame and skin was always earth grounded. More developed campgrounds provide this via the shore connection. For less developed campgrounds they could provide ground connections at each site, with a lot of cost to install and maintain them. When moving down the road an earth ground is obviously impractical. So the code allows the vehicle frame to "act" as the earth ground. What about shock risk? Shore power always has one of its conductors earth grounded, so an accidentally hot frame or skin could give a full-blown shock. Earth grounding is therefore important when on shore power. The portable or vehicle mounted generator has all of its conductors isolated from earth ground, so there should be no current path for a shock to earth ground. There is sometimes a conduction path from the vehicle frame to ground, so the vehicle frame may or may not be connected to some degree to earth ground. If the generator isn't bonded, there's no current path to earth ground. If it is bonded, any current path from the frame to earth ground will put them at the same potential, meaning no current path for a shock.
