Would be interesting to see what the actual discount would be for non use of the facilities. I suspect not much. Cost to maintain roads, campsites, general tree clean up etc would be a fixed cost. Which disabled or not, we all use. Then you would have to figure out wear and tear on the facilities, per number of campers, and the percent on disabled campers who do not use them.
I suspect the cost reduction to the park would be extremely limited. And hence so the discount should be. That would probably be a pretty fair way to go