Tim,
The other thing you can do is plan the interior layout of the van so that the entire side door space is filled. For example put your water tank along side the bulkhead and push your machine to the right so it closes off the door. It would be a brave thief who would clamber over the machine while it is working to nick your spare chemicals.
The water tank would be light and easy to remove giving you brilliant access for maintenance of the blower etc. In my first TM van my Performer was installed too tight to the bulkhead preventing proper access for maintenance. 
All the best
Nigel