Good Sam is 10% (I've gotten that by showing my library card) and Passport America is 50% discount. PPA has been my "emergency" RV Park finder for quite a few years now and when I can't find a campground that's where I go. As others have mentioned that 50% discount CAN be a little misleading if you don't understand it (they give 50% off the regular price for their lowest price site) and if you choose a site with extras, be it extra people, more features or preferred location they don't ALWAYS honor 50% off THAT price but you still get 50% 0ff the lower priced site. There are sometimes other exceptions like holiday weekends, or seasons of the year but these have never been a problem to me. Phone ahead as they don't always keep their membership directories up to date and a phone call will confirm they are still members and could save a drive out of your way.
Good luck / Skip