Every trip I go on I have my spare Bar-B-Q tank in the bed of the truck. It sits next to the spare 5 gallon can of gas with a buffer between them.
Both tanks are tied down to cargo hooks in the bed of the truck and then I run a steel cable through the tops of both tanks and pad lock the cable.
This last trip I dropped the trailer in camp and drove down to Home Depot to get some supplies to make a repair to the trailer. When we came out of the store there was an out standing citizen standing at the back of my truck looking at it. When we got back to camp I found all of the tie downs had been unhooked. Just the steel cable remained.
Bob