Mahoney St Park Camping Question

City boy here, trying to grab 3 camping spots for the family in a few months (July 17th ish). When I try to book online, all of the reservation options are full however half of the campsites are still open but say “first come first serve”.

I’ve heard Mahoney is busy during that time of the year, do I need to show up incredibly early like it’s a Pokemon drop at Sam’s Club in hopes of securing campsites?