I do all the basics first like
I have a partition between the cab and back always locked.
Have every door locked using the factory door locks and added padlocks.
Have no windows and if you do have windows make sure you have a steel mesh behind it.
Alarm with a 2 way pager.
I have several shelves with a steel bar running across it with a padlock as well.
99% of our jobs, we can literally leave the doors wide open and still be safe as we work in great neighborhoods but this is how we store the vans when not in use.
All these steps will slow a thief down, not stop them or change their mind and go to another truck
Another trick Ive seen is if you have an old iPhone connected with minimal service, you can hardwire it to always stay on charge so you can locate the truck if it goes missing. Its a quick alternative to lojack etc