I noticed a pickup the other day that had a toolbox in the very back of his pickup with a dropped center, which allowed for fifth wheel hookup without removing the box. In searching for similar boxes, I came across one website with a box designed in two different ways.
One way is with the normal "top lid" that raises up with three different compartments. The other way was with the center box with a "top lid" but the two sides had doors that opened outward to the rear of the pickup. That one allowed one to open the two sides even if the fifth wheel was hooked to the pickup.
Here is a link:
http://www.pickuptrucktoolboxes.com/...es_unique.html
Terry