Actually, their home page is a bit misleading. That's a surprise, right?
It says they are a gateway
to over 15,000 campgrounds, which is indeed about the number of campgrounds in North America. However, their "member parks", the ones where you can stay for $5 or $10 per night, is listed as "hundreds". See United Member Parks
United costs a lot more than Passport America, but offers steeper discounts at their member parks. Check the park member list or the state-by-state maps of their locations. If you go to the right places and stay enough nights in the system, it may be worth the $189 annual fee.