The first year we had our Casita
we joined Passport America, Camp Club USA, and Happy Camper. After one year and many trips we found Passport America provided the most parks in more places that we wanted. We were always treated well and never were unable to get in, even without reservations. We dropped Camp Club & Happy Camper since we found we used PA the most.
One caveat. Always read the restrictions listed for each park. They vary wildly so be aware. We found the various restrictions made sense for each park involved and were reasonable. Just be sure to read them for each park.
We definitely got our money's worth out of PA. It paid for itself on the trip home from the Casita
factory after pickup.