We have Escapees, PPA, Good Sam, and Thousand Trails.
Before we went fulltime, when we took long trips, PPA and Escapees came in handy and paid for themselves. Our first year fulltiming, PPA saved us $500+. Escapees also saved us quite a bit. We only used Good Sam rarely but it has probably paid for itself.
We stay mostly in public parks which don't offer discounts but need FHU about once per week which is where PPA comes in handy.
Thousand Trails - we were given a Zone Pass for free when we bought our rig. We have used it for three days so far and plan about 6 more days before it expires. We will not renew. All of their parks are also in PPA. If we stayed in the same place for a couple of weeks, it would probably be a better value. at $500/year for one zone, it just doesn't pay for itself.
Escapees is a club that offers much more than just campgrounds. But, if all you are interested in is campgrounds, they have two, which are quite nice, in California. One is about 25 miles from Yosemite and you can't beat the price for the location.