Safe Transport/Storage of Aux LP Tanks?

Federal DOT regulations prohibit at least permanent LP tanks from extending in front of, behind, below, or above the structure of the vehicle. (As I recall extending beyond it to the side was not specifically prohibited, I suppose because just about nobody was crazy enough to attempt that permutation.) I am not sure whether or not those DOT regulations extend to cylinders that are "temporarily" mounted on a vehicle, but common sense would suggest that it's still a good idea in as much as the basic danger in a collision still exists.

If there's room on the trailer tongue, that would be a much better place than hanging off the back of the trailer, in my humble opinion.