I did something similar with 2 battery boxes, 1 mounted on each side of the spare tire. I use those for wood leveling blocks, so there is not much weight. I also mounted them as close a possible to the camper, so the weight is not hanging way behind the bumper.
You will need to keep a close eye on the bumper welds and be aware of
the weight added to the toolbox, so your bumper doesn't end up in the middle of the road somewhere.
Chap , DW Joy, and Fur Baby Sango
2006 Ford F150 Super Cab 4x4
2008 Surveyor 263