We have the KOA card and a Good Sam's card. Also, a lot of parks will discount with a AAA and AARP card which we also have.
I'm not into clubs like Thousand Trails, etc. I don't liked to be locked in as I recall T. Trails will keep charging you until you can sell it (if that's still the case).
It seems like the best located parks don't take discount cards. Prevost has a deal with the Las Vegas Motorcoach Resort which we frequent often. I think they also accept FMCA cards, or used to.
One thing is for sure, I wouldn't pay for any expensive 'membership' club.
Safe travels,
MM.