Membership campgrounds can work or not, depending on what you are looking for and your style of travel. Some people like having a few fixed places to go, getting to know the other campers (with a similar outlook) and maybe participating in organized activities at the member park(s). Others prefer to travel and see different places, in which case it may be difficult to get your money's worth from even a chain of membership parks.
If you are thinking of buying into a single or chain of sites, look for resales - they are widely available and nearly always much cheaper than buying from the resort itself. Ebay is one source, but there are resale brokers who handle them as well. Google 'campground resale' to find some of them.