The best security I had was a Bullmastiff and a Great Dane.
They certainly put off the opportunist thief on more than one occasion, both times in service areas when the person showing a lot of interest in the van hitch looked up straight into the face of the Bullmastiff and being looked down upon by the Great Dane..................he suddenly decided he had a place to be,,,,,,fast!
The Milenco unit as linked by Parksy looks good, similar to the Fiamma door lock but with the advantage of being able to lock from inside the van as well.
If your intention is for security when the van is unattended, then in reality if they want it.............they WILL have it, regardless of security.