RV Park Discounts are possible at all of the parks on this list. RV52 keeps track of which parks in its massive RV Park directory have memberships with Passport America.
This particular list is a list of Passport America RV Parks. You should be able to get RV Park Discounts at these parks. I've attached a link here so you can get your own RV park discounts.
This is a pretty big listing. The best way to find a park in this list, or one close to where you need to stay is to look for your two letter state/province abbreviation.