These no reservations campgrounds that have poles for your tags....you are usually allowed to confirm a departure in a site you want, go ahead and pay for however many number of days you want, then slip your tag beneath the one of the folks that are leaving.
You can then take possession of the site as soon as the current occupant departs.
In very popular, high demand campgrounds, it can get a bit cutthroat, but the early bird gets the worm...or site, as the case may be.
Maggie