My youngest son is 14, about a month younger than your oldest DD. We wouldn't think a thing about leaving him alone in the RV for a couple of hours or so. He started staying in the RV alone while we took 30-60 minute walks (often away from the campground premises) when he was 11 or 12. In just a little over a year he'll be a licensed driver. In a few very short years he'll be heading off to college, perhaps thousands of miles away. The time to start learning responsibility and gaining independence is now, not then. Of course all the common-sense things apply: He has a cell phone, campground feels safe, etc.