14" is an insane amount to make up for clearance. Example. An F-350 bed is just over 20" deep. I don't see how you can do this.
I assume your jobsite box is intended to be lifted into and out of the truck on a regular basis. If that's the case, that's the answer. But if you need a way to perform this task alone, read on. If the toolbox is permanently mounted in the bed, read on.
Wild speculation - and a bit tongue in cheek:
1. Lift your rig. Add a lift kit to increase ground clearance on the fivere by, say, 3 or 4 inches.
2. Convert the fifth wheel hitch to a gooseneck...assuming your rig's pin box will accommodate that switch. Some will and some won't.
3. Extend the pin box to move the rig rearward, once again assuming the pin box can handle that extra stress.
4. Do the math on the measurements and see if you can make the needed turns and maneuvers with this contraption.
I don't think it will work unless the front of the trailer is VERY high compared to the rear.
Alternatively, and less crazy...perhaps...make your job box more easilyh removable by one person.
Change the mount to some sort of quick release. Perhaps bolt angle to the bed forward of the box. Drill a hole to
receive a pin bolted to the side of the front feet. Match the pin to a hole in the angle iron. Add
clevis pin or hitch pin.
Add
casters to the bottom of the box to wheel it clear of the cab.
Weld on or
bolt on hoist lift points to either side of the box.
Now you have a quick disconnect tool box that can be rolled to the rear of the TV bed and lifted out with a
floor crane.
You still might want to look into that
Gooseneck adapter for the fifthwheel pin box and adding a gooseneck ball in the bed rather than having to remove or install the fifth wheel hitch every time you want to tow the rig.
Or, nice as the Knaack is, find another way to carry your tools.
Again, largely tongue in cheek. I don't think your Knaack is worth the effort, unless it's routinely removed from the truck and locked down on a job site. Perhaps you have workers who perform that task. If the Knaack stays put, there are lots of bed-mounted toolboxes that stay below the bed rails of your TV and leave room for a fifth wheel hitch...and you can sell the Knaack to recoup some of that expense. Every other option is FAR more costly.
Best of luck.