I've done a combination of backpacking and tent camping from motorcycles, cars/Jeeps and now vanning since the late 70's.
The AT, state and national parks, private campgrounds, state campgrounds and in my youth private lands I probably shouldn't have been on.
I've never had anything stolen.
I attribute that to a few things...
I try to stay away from people, away from other campers in campgrounds, away from campgrounds in state/national parks.
It's a lot scarier for a thief to walk around in the pitch black to my secluded/remote campsite and he's more obvious as "not belonging around there" in broad daylight.
I try not to bring anything of value that I'm not personally carrying.
Wallet, cell phone, vehicle keys, etc.
High value items like a tablet, good camera or laptop, get hidden away in an inconvenient to get to place, not left out in plain sight.
I try to make the camp look like someone just left and could be back at any moment.
If your camp chairs are all put away, cooler under the picnic bench, the tent zipped tight, the umbrella or awning closed and the clothesline empty, well you are clearly not home and are going to be away for a little while.
I've left small FM radios on inside a tent or vehicle, shades drawn so people can't be sure someone isn't around and just taking a nap.
I use a good locking hitch pin and lock BOTH safety chains to the vehicle. (Note: there is an appropriately rated shackle on the chains that actually takes the load if needed, the locks are security only not load bearing.)
If I have to park the trailer without the van, I use a locking hitchpin, one of those cast aluminum ball locks with bar and a high security hasp lock, a motorcycle high security chain and lock through one wheel and a long cable through the mounted spare tire and other wheel. Sure someone could defeat all 4 locks, but they will hopefully look for easier pickings.
What else could you do?
I have set triplines with fishing lines and bells/cans, but found I was only waking myself up to see nocturnal 4-legged creatures scampering off.
With small wireless cameras becoming more and more prominent, just putting up a "Smile You're on camera" sign might be enough of a deterrent.
Leave a set of windchimes out and the chairs in the usual spots. Or tack a note on the table or vehicle..."Hi Carl, I'm walking the dog, back in 5 minutes." So not only are they now having to keep an eye out for you returning with a dog, but also Carl.