One tip I can offer is, to buy a couple of personal attack alarms. You can get them from any good DIY outlet.
They are the type you attatch to baggage (hand bags, brief cases etc). Sorten the pull out cord and replace with 4LB fishing line.
Stretch the line out either across the doorway, or by fitting the alarm under the van, to the ground.
You get a 120db protable alrm, and the trip wire is virtually invisable. This might give you a last ditch nuisence to the thieving B*****ds when they strike.
I'm afraid you won't stop a dedicated proffessional, but you might make it too awkward for him and he'll buzz off to easier pickings.
I park my twin axle front facing in on the drive, have two wheel clamps of different design, a hitch lock, mains plug lead removal protection (voltage drop sensor) and I park an alarmed car inches from the van covering the wheel clamps. Oh and a post locked in the ground at the road side.
There are other gadgets fitted but I don't want to disclose those as (A) everyone would know!! and (B) they are also illegal.
I know of one incident where a guy got up at 7am to go to work only to find his caravan half way out in the road. The thieves had over come the alarm and hich lock and wheel clamp. Taken out two fence panels and dug up the fence post. Only to be thwarted by the freshly dug flower bed covered in wood chips. Good job his wife was on old school gardener and "double dug" the garden.
A 3 foot by 18 inch deep wood chip covered soft dug trough was what foiled the little s****. It took a couple of hours and a few air bags to lift out the van which had sunk to the chassis in the mud, but atleast they kept their pride and joy.
You see each individual item of security add's minuits to escape time and this is precious to the thief as he wants your van not a free room with iron bars for a few years.
I must admit to liking the arabic way (taketh the hand that doth offend)at least he'd only do it twice!!
Steve.