I lock my RV when I'm away. I don't generally leave much sitting out around the campsite, not so much for fear of it being stolen, but rather because I don't want to take a lot of time to set up or tear down camp and because I don't care to sit in a wet chair or cook on a wet grill after it rains.
I have had a welcome mat and homemade leveling ramps taken (the only items in the site) when I was out with the motorhome touring one day. I suspect it wasn't foul play, but rather a camp host/volunteer not realizing the site was still occupied. For next time, I guess I should leave some chairs and the BBQ, or else take everything!
(As to what tent campers do, if they're like me they lock up valuable things in their vehicle, while leaving stuff like sleeping bags and the tent out.)