I think it's safe to say that an RV wastewater tank and related plumbing are designed to withstand being fully filled and driven to a very distant RV dump station with no damage whatsoever. Otherwise you'd see failures routinely, and there'd also be a warning in your RV owner's manual. And like home sewage plumbing, RV wastewater tanks are normally (or at least often) vented via pipe to the RV roof for odor minimization, so I don't know that graywater "overpressurization" is even possible.
As for dumping graywater on the ground, I'll defer to "Just because you can get away with something doesn't mean you should."
