The idea of a *deterrent* to simply to slow them down, or make them think twice about touching your van. Stickers will do that. Unfortunately little is guaranteed to actually *prevent* theft. But if a question of doubt is raised and they move on to the next one, it was worth it.
I like the idea of the gps unit and have have it on the to list for a while, but only addresses the issue after the fact. In my case, (as was Jay70's), where my van is stored in a yard, it would take me 15 minutes (illegally) to get there if a motion sensor was triggered. So someone could still get in and take something if they wanted and be gone before I got there. (the yard is alarmed, key card accessed, multitude of cameras, and has a live-in guard so not too concerned in that regards).
Theft while traveling is another issue though. We can leave a van on a site and be doing the tourist stuff and be 50km away and I couldn't do a thing about it if someone of the scum class broke in.
I've been thinking of a car alarm on it, combined with the gps. A screamer would draw attention of anyone around, and being a car alarm can be triggered by movement, doors or boots opening. Add a flashing light or two
????
Maybe were could get some professional looking 'brand-name' stickers made up. ?? Some professional custom stickers rather than off the shelf stuff could be better
I always remember my dad going into Tandy and buying a bunch of 'This house is protected by Radio Shack' stickers he put on every window. Tandy never changed the design for years. Even though I wasn't even 10, I knew you could just buy the crappy looking stickers by themselves (different to those which came with a security system). If I knew that, a theft certainly would.
edit ; although just looking through Jaycar catalogue there are a few simple ones in there which don't scream "just kidding'