With National Parks and State Parks, they all filter through the rec.gov reservation system, so those are straightforward systems with guaranteed reservations if you make them. With private campgrounds, it's different. Each one uses a different method, and some use third party sites outside of ours. Beyond that, there are others, still, that like the old fashioned method of a telephone call and take reservations on paper. There is currently no good way for us to incorporate all of those methods into one system.
For the best user experience, and for the benefit for campers that rely on having solid reservations when they arrive at a campground, the very best option is for you to call the campground directly or visit their website so they can tell you what availability they have. Especially with the ones that are first come first serve, and don't take reservations at all.