I understand this is an old thread but the following info may be of use.
As a COSHH assessor I would take into account what chemicals you're using, whether they need to be locked up and how many people have access to them.
If it was simply a one-man operation, then there's no need as long as others aren't using the van. If the van is used by others and they're in an accident then the last thing you want is a bottle or tub of caustic exploding and finding yourself being held liable.
For what it's worth, if you're using caustics of any sort then a cabinet can be bought relatively cheap and give you peace of mind.