If you qualify you should get the National Parks pass. Even at $80, it will pay for itself in short order. 50 percent off overnight fees at national parks.
KOA has a discount program. Unlike Passport America, they all honor the program.
RVillage is a social site that allows you to connect with people in the area where you're camping. I've met a few people through their site.
We also joined FMCA for the benefits - things like discounts on Michellin tires, emergency assistance to get home after a catastrophe etc. Finally we joined the Safari International chapter to connect with other Safari owners.