Always check your van doors are locked before leaving your vehicle unattended
If your van is parked on a driveway, consider installing security lighting
If you have a garage use it
Consider alarming the vehicle internally
Move what tools you can out overnight - hopefully at least the highest value items
Visibly mark your machinery and tools using an engraving or chemical etching kit or use a forensic marking kit. Place a sticker in your window to say you have done so - the signage alone, can be a very effective deterrent.
Keep a list of tools, together with serial numbers and any identifying marks. You can do this at www.immobilise.com. This will help to trace them back to you if they are recovered.
Even if insured, the inconvenience of having to replace tools can result in several days lost business to small tradesmen, causing a loss exceeding the financial value of the items alone.