When I've carried bikes on roof or hatch racks I always run a long cable through wheels and frames and rack and then I lock to the vehicle. My van has tow loops under the rear bumper and this is where I anchor. If I plan to be out of sight of the vehicle I remove the seatposts/saddles on a bike with seat QRs. You can find 10', 15' and even longer woven cables with end loops, and get yourself a massive padlock. I'll carry the bikes inside the van when possible, but luggage and passengers prevent that sometimes.
Just be as secure as you can and try not to worry too much. If someone wants your stuff badly enough there's little you can do to prevent it.
This is not a great photo but you can see the lock on the tow loop at the bottom right, and the cable snakes through frame, both wheels and rack.
Camper loaded.jpg