I believe you need to go to Reserve America site on-line, locate the campground and book from there. It is EXTREMELY competitive to get a site - they are week long bookings and they book within seconds because it is a popular campground.
Do you want a good site or just a site you can reserve for a few days or longer. Go on line check availability. Sometimes last minute res. may mean in order to stay for longer than a day or two you may have to move from one site to another.
Thanks! Your answer is awaiting moderation.