Got the tool box mounted today and it went smoothly.
I used the following parts all of which were purchased from Lowes for about $46 including the toolbox. The pic shows where I cut the L-brackets to the length I needed.
2 1/2" U-bolts
4 1/2 flat washers
4 1/4" bolts (1 1/2")
4 1/4" lock washers
8 1/4" flat washers
Bostich toolbox with weather seal
I spray painted the L-brackets and the flat bars from the U-bolts and mounted to the bumper. I test fitted the brackets and marked the bumper with a white paint pen prior to mounting them. Once they were mounted, I cut the threads down since they were several inches longer than needed.
Once the brackets were mounted, I placed the tool box on them and marked the bottom where the holes needed to drilled to mount it.
The tool box was secured by using the 1/4" bolts and the flat and lock washers from the inside. The bolts are mounted in the corners out of the way and don't interfere with the large storage area.
The tool box has a small tray but allows plenty of storage underneath it.
I am using the tool box to store a spare 90 degree fitting, the dump station adapter w/ donut and box of gloves. Everything is now stored close to the dump valves and won't be in the pass through storage anymore.
The camper is a hybrid with a rear bunk so I was limited in how far over I could move the spare and place the tool box so they wouldn't block the bed support poles.
The longest part of the project was taking the L-brackets to my buddy's shop to used his grinder to smooth the corners and edges. Once that was done, it took less than an hour and was inexpensive.