Man, we came so close to getting burned not making reservations. We were going to the mountains, like we had done many times. But, it was a big weekend, like Labor Day or something. We got to the campground and they were sold out. But, someone on one of the long term sites have moved their camper for the weekend, so they allowed us to stay there. We never travel without reservations.
I would also agree that there's not enough campgrounds. For example, Wilmington NC. The only campground there is the KOA and they are landlocked. It's a great campground but, you better make your reservations months out, a year out or more for holidays. In the off season we have driven to Myrtle Beach without reservations because there is plenty of open spaces, once or twice. I tend to still make reservations just in case something comes up and we roll in after hours.
I have often thought that owning a campground would be a great idea. If you had property within a short drive of Wilmington it would be a gold mine. There is a small campground in Carolina Beach but, it gets bad reviews.