Most rear bumpers I have seen are very thin metal, and are risky in supporting much over a few pounds. I would take an end cap off and look at the end thickness of the metal. If it is less than 3/16" thick, I would not recommend mounting your spare tire on it. While it may seem solid when at rest, you have to not only consider the actual weight of the tire and bracket, but also the bouncing stress on the bumper when going down the road. Also check the strength of whatever the bumper is mounted to. An alternative would be to have a receiver hitch welded onto your frame, and use a removable carry-all that slides into the receiver. JMHO.
2003 Ford F-350 V-10 Crew Cab 4WD Long Bed
2004 Jayco Designer Medallion 29 RLTS 5th wheel
Bill, Gayle, and Mindi (dachshund)
Retired at last !
Nothing is foolproof to a sufficiently talented fool.