We don't stay at campgrounds often, but this last weekend we were at one, and we had a site reserved at a different one starting today in the South Dakota state campgrounds system. ( After not being able to get boondocking sites last year, the DW said never again.) We were going to be at the new campground at 12:30, and checkout is 3:59, check in is 4:00 pm. I was totally prepared to sit for a few hours, but lucky for us the site was empty when we arrived. Before anyone starts whining, if the site is empty in South Dakota, you can have it, if you have a reservation. If it is still occupied, they "own" it until 3:59 pm. at 4:00 pm, it is whoever has rented it.
Yes there are people who arrive early as we did, and try to pressure people to leave so they don't get inconvenienced, but you can tell them to pound sand. I know of one campground host who will also pressure people to leave if they have friends waiting for a site. They always get poor reviews from me. They are the type of host that don't want to be bothered, and do not clean up anything in the sites when the previous people leave.
I had no intention to even say anything to the people of the site we were going to today if they were still there. I just assumed that they would be out fishing, since that is what most people do at this particular campground, but lucky for me they weren't. It was a good Monday! :)