BigH,
The reason for chaining the case down is to prevent it from being "thrown out" of the truck bed during an accident, and to prevent theft of the gauge (and case) from a typically open truck bed (truck bed caps don't do much, if anything, to mitigate either of these problems).
Unless the case is stored for transport in the locked "boot"/trunk of a car (i.e., not readily accessible to theft nor able to shift about during transport), I would personally make an effort to at least tie the case down inside a van to a secure anchorage point to prevent it from moving around.
Due to reports of recent losses (in the past year or so) of gauges that I am aware of, the various state regulatory agencies take such losses of radioactive material very seriously indeed and can get quite animated.
Jeff