I am assuming that the toolbox is behind the hitch. No toolbox here but have one of those cutout type tailgates which amounts to the same thing when it comes to kingpin clearance when backing in. I can't see the top of the tailgate cutout.
The guide is that if I can see all of the kingpin in the mirror as I back in, I won't hit the tailgate cutout top.
Maybe same with your toolbox. If you can see all of the kingpin but not the toolbox, you are safe from hitting the toolbox.
Whether that is high enough for the hitch is another matter.