What are pole using to mechanically secure their vans? I am picking my 535GT up soon and wanted to see what people are finding the best products. I will probably go with the Al-Ko hitch lock but wondered what people are using on the wheels?

Don't forget the windows, you can git through them in 30 secs without even damaging the window.
Dave

Is there any way to protect the windows? Was it on here that someone made an ingenious thing with a tube of plastic that stopped the window catches from opening?

Take a look at Millenco window alarms and ‘lock-em-out’ window catch locks.

It depends on what you are trying to prevent. Theft of the whole van is always possible, but you don't see too many of them in the convoys of those folks who like to do a lot of traveling or in the garbage infested dung heaps they leave behind after trashing the area. Portable angle grinders will deal with pretty much any normal security device/wheel clamp etc. Park nose in on your drive if possible, block the van in with car or fit a security post.
Opportunistic entry to nick whatever can be nicked and sold for crack etc is far more likely. All you can do is make it harder for the slimebags to get at your stuff. Don't leave chargers, phones, tablets cameras etc visible. If I had a TV I think I'd be concerned about the aerial as that kind of advertises " Here be loot me hearties". Net curtains plus fly screens are really useful for obfusticating the view into the van.
I don't know if alarms are of any benefit. Does anyone pay attention to them? The best ones are those that totally destroy the hearing and wreak permanent damage on the intruder but that can be a problem if you set it of by accident I guess.

It's really a matter of common sense. As has already been said you can't prevent a break in so don't leave expensive stuff in the van. when it's stored. Not so easy when on the road. The advice given is sound. Don't make it easy to nick when stored. Our little Puck sits out side the house in full view. The tow hitch is blocked,a gas bottle chained to one of the wheels. All very visible. We always close the gates at night.. If I was worried I'd think of jacking it up, removing the wheels, maybe also the brake drums, and keeping them elsewhere.