Only camping theft I've experience was about 40 years ago at the parking lot for the Snow Lakes Trailhead to Enchantment Lakes, along Icicle Creek Road out of Leavenworth, WA.
We left our VW Beetle at the parking lot for about 4 days or so while we backpacked into the Enchantments --- after first checking at Leavenworth USFS ranger station for any reports of animal issues in the area.
In retrospect, should have asked about people issues at the trailhead, too.
When we came out from a great trip into the Enchantments, we discovered our Beetle (which we had left locked) had been broken into, with the vent window jimmied open and the glove compartment accessed to pop open the front hood. Our luggage had been rummaged through (we were on an extended trip that included continuing to North Cascades NP for another backpacking trip after we came out of the Enchantments).
We were lucky the VW could be "broken into" without breaking anything.
There were at least two dozen additional vehicles parked in the parking lot, and every one of them had been really "broken into" with windows smashed out in every one of them.
A regular crime spree in a trailhead parking lot immediately off of a highway.
Returning to Leavenworth, we first called our credit card companies to report that our credit cards had been stolen.
Then we went to the Leavenworth USFS station to let them know there was a problem at the Snow Lakes trailhead parking lot. Response? Yep, they said, there's been a problem with break-ins at that lot all summer.
Well, gee, I thought to myself, thanks a lot for the non-warning earlier when we had checked in before our trip.
Upon going through our luggage later that day, we found the culprits had overlooked our credit cards after all. Unfortunately, that discovery was too late since we had already reported the cards stolen and couldn't use them for the rest of our trip.
At another time, a few years earlier, I had reason to appreciate the honesty of folks who actually hike into the backcountry. My camera got "forgotten" on the ground at the start of a climbing pitch in the Tetons. Some nice person found it and left it at Jenny Lake ranger station -- where it was waiting for me when we checked-in after our climb.
Wouldn't be too surprised, though, if the increase in folks hiking into the backcountry these days doesn't include some who aren't so honest -- at least when it's within a day hike of the trailhead.
That's unfortunate, but seems that's the way it is nowadays.