We normally leave something cheap behind as well, like a plastic tablecloth and a jug of water or something like that.
We have only had one problem, in Galvston, when someone took our site when we went out to dinner. We got back late and found a tent and 2 cars in our site. They refuesed to move "because the kids were already in bed". The campground hosts were nowhere to be found so we pulled into our spot and slept there anyway, since we knew it was our vehicle license that was registered.
The intruders did pack up and leave the next morning. I was a little ticked when the park ranger came by later and tried to make us pay up for having 2 unregistered vehicles and 2 tents in our site, but we set him straight and they wound up giving us a refund for what we had paid. They had the license numbers of the 2 cars and were going to send them a bill - with a fine for not having registered or paid on the way out.
That same day we got a "Site Occupied" sign from the local campstore and have not had a problem since.
A good traveler has no fixed plans, and is not intent on arriving. ~ Lao Tzu