Just a few thoughts and please don't take any of them as teaching granny to suck eggs.
Security is all a matter of cost versus return, if it costs you £2000 to secure you £1800 kit whats the point you might say.
Well its not that simple is it?
You have to consider the higher insurance you might have to pay, the loss of your vehicle, the loss of work and most importantly the loss of reputation when you can't carry out work that you have committed to.
Going back to basics you have to remember this simple fact, there is nothing on the planet that can stop a determined thief with enough time.
So there you sit with your nice signed up van saying come and get me, which might also have a little sign saying "no equipment left inside overnight" which no one will believe.
Good security is three things, its in your face, it looks the part, and it slows people down.
Getting to specifics good quality locks should be on all doors( if you have padlocks use abus type discs)
If you have a sliding side door put a good lock near the base on the inside to prevent the door being forced off the slider from below, if not doing this always have the inside firmly wedged with a length a solid timber to stop opening if main case lock forced.
Steering locks should be of the type which no matter what the thief does the wheel will not turn enogh to allow steering and they should be very obvious( Halfords sell own label wheel bars for about £40 and they work)
Finally if you are really convinced that your veichle might go walk about, get a denver boot
Remember that thieves are opportunistic as well as lazy and if you make taking you kit difficult they will go elsewhere,if you make it very obvious that you have taken time to delay there activity, they will go elsewhere, thieves like the rest of us are in business and if the risk is to great they will not take on the job,
i hope the misive as it were has been helpful if you want something more specific pm me