If you need a home address they are an option but not the only one by far. In fact, many of the marine ones you may have used are quite viable.
Unlike Yacht clubs, you don't see as much of the formal groups camping. I think it has to do with most people stay most weekends at their home marina so you get to know each other. Most campers are kept at home or in a storage yard and everyone heads out in different directions on the weekends.
Most of the clubs we've come across are usually groups of friends that have grown organically rather than formal clubs. Eventually, someone takes on the role of making reservations and organizing dinner but not like a yacht club.
If you are looking at discounts, yeah, GoodSam/CW work. Another to consider if you are moving a lot is Passport America. They are mostly for short term stays (1-2 days) but really good deals. If you move a lot, they can save you a lot. If you tend to be more stationary only moving once every month or two, they likely won't save you enough.