I just wanted to add our experience with resort wheelchairs. The ones I've gotten at OKW don't seem to be in as good a repair as the rental units at the parks. Whenever we travel without bringing our own, we go ahead and pay the daily rate when we get to the parks, plus you don't have the hassle of loading them on and off the bus or in a rental car. If the person in need of the chair is fairly ambulatory & able to get to and from the park entrances OK, then I'd rent at the park. If you do need one to use full time, I'd rent from an outside source.
Be sure and pack your handicap placard if you'll have a rental car.