I am just a weekend camper that used public campgrounds in SW MN, Northern Iowa and, Eastern South Dakota. Check in and out times around here are usually 4 pm.
I did once get to a park on time once and that there was someone in the site. I waited patiently for awhile but, noticed that there was no one at the site. There was no park workers there at that time but, there was a phone number on the door to call. I called it and they were shocked that the other camper was still there. Long story short, the teenager they hired to work the desk screwed up and double booked the site. They offered me a different spot, one free night of camping, and free firewood for the weekend. I was happy with that. They were apologetic but, I just said S##T happens and I was not upset.