Back in the day when I towed for a living I considered this, until I realized it would be in the way of a gooseneck. So don't use a gooseneck? Fine. The other problem I found was like what Bingo states that a V-shape boxes tended to have several small areas of storage and most of that I wanted to store in those areas were too large for the compartments.
I was very close to purchasing a shorter in height rectangular shaped tool box much in the shape of the large tool box at the truck bed next to the cab, but then I quit towing and therefore didn't have a need. It also would have been in the way of a gooseneck, but at that time I was okay with that.
In this link it is called a "partner box" to the fifth wheel (not above the rails) tool box.
http://www.800toolbox.com/5thwheel/5thwheel.htm