Been camping since 2003 and only one time did anyone attempt to steal anything and they weren't successful.
We were camping at a state park at the Guadalupe River in Texas. We had a pop up back then. We had our ice chest under the front bed end. I was awakened by whispering voices under the bed and some other noises. I woke up my husband who listened for a minute and said "kids". He then yelled in his loud dad voice and said "Hey you better not be stealing my beer". With that we heard a head hit the bottom of the bed... hard... and a kid say "dang" (well you know what he said). My husband headed to the door in time to see one young teenage boy limping away (must have hit the hitch pretty hard, is our best guess) and another one running while holding his head. We found a few beers scattered by the ice chest and a couple of things knocked over but nothing was missing as far as we could tell. Funny how the brothers a few sites over never looked our direction for the rest of the weekend and had been friendly before that. LOL That memory brings smiles to our faces.
We leave chairs, a table, and other accessories out all the time, even if we are gone in the boat for several hours. Nothing has ever been stolen. In some campgrounds don't even lock the motor home because nothing worth value (other than our dogs) is left inside. If something happened while we were gone we think the dogs would have a better chance to be helped out if the door wasn't locked.