What I have done is I go to web site to reserve a camp site for certain days. Then I look at the overall campground showing what sites have been reserved for those days. If there are a lot available, then I don't worry about making a reservation. If it is getting full, I will check the same place a few days later and see what my options are.
I have reserved a particular site for a state park once, and it was very difficult to get into. When I got there, plenty of better sites were available, so I never wanted to get into that situation again.