All the RV parks and state parks I’ve camped in have been safe. People leave their bikes out unlocked. BBQs, folding chairs & tables, and whatnot are also outside and unprotected. I do the same and have never had any problems.
I’m presently at a campground in Bar Harbor, Maine. A neighbor here leaves the door to her travel trailer open all day while she & her dog go off somewhere. I’ve been here 2 weeks and the only time she closes her door is at night. An RV park like this one has a lot of visitors and staff going about their day, so a crook would probably shy away from “working” here.
I was previously camping at Wompatuck State Park near Boston. The campsites there are much more secluded than where I am now, so common sense would lead you to be more cautious anyway. Even so, as I said earlier, I’ve never had any problems while camping at state parks.
Consider adopting a dog for companionship, as a way to meet people, and to ward off potential bad guys. If you don’t like dogs, get yourself a BEWARE OF DOG sign and stick it in your camper’s window…think of it as insurance!