I agree you need to pay for the site then save it. Most campgrounds we go to require a tag to be placed where it is visible to show the site has been paid for.
An alternative would be if someone was there to watch over the sites for friends on the way to the campground. For example a club or group of friends. The first one in the campground pays for their site and then places chairs in other sites for their friends to make sure they all get to stay in the campground. But they must stay in the campground to watch over the chair-reserved sites.
At one really popular first come first to get site campground in California folks were actually reserving the sites with their RVs and then selling the site on Craigslist or a website. When the sub-renter shows up the "local" will move their RV to another site. People who lived nearby found a way to make some extra cash by sub- renting the sites. The officials have put a stop to this practice.