When we had our class B, and now with a small C and no toad, I would/will not leave anything valuable in a campsite while we are out and about. We often put a cheap tablecloth on the picnic table. It's really annoying, though, when people scarf something useful like a rubber door mat since we have to go to WalMart again and spend the $5 bucks.
The biggest concern for me is losing the campsite itself in "claim your own site" locations where just a slip of paper on a pole indicates it is taken. Or in any type of campground where people decide they like your site better than the one they were assigned and see an empty one as an invitation to move.
For several years we've used a red traffic cone marked "reserved" plunked down front and center at the site entrance and so far no one has swiped the cone itself.