I have been doing some investigating about Campground Memberships. Passport America will give you half price camping at over 1600 campgrounds in North America. Then there is the one that is affiliated with Camping World, called Camp Club USA. I think that in this category, Passport America wins as they have numerous campgrounds in Canada as well as the US.
Then there are places like Coast to Coast and Resorts of Distinction which have a large membership fee, but then you don't pay to camp.
So many choices. More investigation is needed.