Would the casters hang down lower than the existing skid plate? It seems to me it wouldn't take too much for the weight on them to become considerable, particularly if you have the coach loaded. That could be a lot of force on the trailer hitch and casters and over a small contact area on the driveway.
I'd go with the supplemental air bags, assuming there's no way to alter the approach angle when you drive into the driveway so as to lessen the transition—either by regrading the driveway (probably not an inexpensive option), or by driving up it somewhat more obliquely, or something of that nature. They might also improve the ride going down the road a bit.