If you go to modmyrv.com and look at exterior mods, you'll find one where a trailer owner had two custom diamond tread boxes made and mounted them on the rear bumper. I don't know if I could post the guy's email address here, so go to that site. There is an old link to an ebay page with the guy's name and number that builds the boxes. I'm having him build one for me to very precise dimensions so that it takes up half of the rear bumper and goes up to just under the taillight. It's going to be tall enough to carry my air compressor along with some other heavy stuff that I didn't want on the tongue. The price was also very good, $250 plus shipping to me. You can't go to Northern Tool and buy one for that price, and a custom built box solves a lot of issues.
If you have trouble getting the contact info, PM me and I'll send it to you. BTW, he's building mine so that the door opens by swinging down. The design is called "under truck". I'm having the width made so that it doesn't stick out past my spare tire. MY shore power connects in the middle of the back wall, so I'll have the box on one side of the inlet and the spare tire on the other side. Not symetrical, but it's what I have to work with.
2013 F250 XLT
I now have a Keystone Outback, but I try to help when I can.
Bitter Gun Owner