I go to public dry campgrounds, not RV parks or private campgrounds so my experience is colored by that.
I have never arrived with my reserved site occupied.
I have never had the next occupant arrive while I was still at the site.
If arriving on a Saturday, Sunday or Monday, I try to make sure I don't arrive before 2 pm as campgrounds are full and I don't want to come in while others are trying to get out. Even if my site is empty, the weekend exodus can make the campround roads crowded.
If there is a reserved tag behind mine (showing that someone has the site reserved the day I leave) I try to get out by checkout time. If it is not reserved and it is midweek, then I may stay up until checkin time. This has never been a problem. But I am careful to check on the campground's occupancy - if it will be crowded, I get out earlier to again avoid the crowded roads.
One time, upon arriving at my campsite, a couple said "oh, we were hoping you wouldn't arrive yet. We wanted to get a photo of us at the water." I explained to them that my campsite didn't extend that far, pointed out the boundaries, and told them they were quite welcome to take their photos. That's the only " arrival issue" I've had.
One time, on a Saturday morning a 5er setup next to me. They were completely unpacked by about 11:30 am. A bit after 2 pm, a large trailer arrived and I heard a lot of back and forth and saw the camphost come over. Apparently, the 5er just "took" the reserved waterfront spot knowing it wasn't theirs. They arrived early so that they would be fully unpacked by the time the rightful camper arrived. The rightful camper ended up taking another non-waterfront site to avoid a big fight. I'm not so sure I'd be willing to let my selected site go that easily. Especially as the crashers knew what they were doing.
If it were simply a case of the previous occupant hanging out later, I'd first talk to them and see what was going on. Maybe they had a problem - engine trouble, missing kid, injury, etc. If it was an ongoing problem, I'd try to be sympathetic and understanding and work with them. If it was just that they wanted to squeeze in as much time as possible, I'd give them 30 minutes to pack up and head out before bringing in the camphost AS LONG AS there was an overflow area for me to park in.