We joined Passport America for a year to check it out but we couldn't find enough participating places that we could use that it never really worked out. If you stay in a participating park it can work out well, but there weren't any where we went. We would have had to go out of our way to find any, and a couple that we did come across weren't someplace we wanted to stay very badly.
KOA works but you discovered the downside: they're high to begin with. Good Sam works, and the discount is honored at lots of campgrounds.