A good hole saw and drill couple of holes in the concrete slab, then cement in a Hi tensile chain or bury a couple of 500 mm star pickets into the ground with a length of fat hi tensile chain welded to them .And good locks " Attached in an awkward spot to get at " .
Either way a battery operated grinder would cut open / off most anything pretty quick for a determined thief ..
Have good insurance and a GPS tracker fitted to the camper .
Ohh and secure those safety chains would help too
https://www.youtube.com/watch?v=Z9tbSzWkY1sThink the gas bottle holder needs to be worked on a bit too . Bit loose, when that dodgy bloke gets the trailer home .... Is it fixed yet