I had an experence like "Turbo" had (posted at beginning of this thread). I had hauled a dirt bike several times from south Louisiana to Arkansas (about 8 hours drive) and no failure, then on one trip home we took a different route which was a lot bumpier. Hwy 65 heading south bumped us for 6 hours. The rear bumper on my Keystone VR1 was maybe 14 gauge thick metal (this so you can slide the sewer hose inside a 4" square tube).
Problem is like most have posted here, it is barely strong enough to hold a spare tire much less anything heavier. Lucky for us we caught this before it came loose at highway speeds. The only reason it didn't completely fall off was I had originally added a 2" receiver under the bumper (to be able to haul a cargo rack, bike racks, or tandem pull my small utility trailer) and had welded the long hitch extension to the trailer frame. That 2" extension tube in the center of the bumper (which was welded to trailer frame) is what held the bumper from completely pulling loose.
Note: I had also tandem pulled many times without issues Using this same setup (Did that several years and inspected the bumper and never found any cracking). The 2nd trailer was more of a constant weight and didn't have a twisting effect - It was the bouncing and twisting action the bike rack exerted on the square bumper that caused it to start cracking. Therefore I don't recommend hanging a bike rack off the back without adding (welding) additional bracing, or simply replacing that wimpy bumper with something way stronger.
The solution for me was to replace the entire rear bumper with 1/4" wall square tubing. And weld it good (not like the factory did with a wimpy weld!) Now the new problem is that with the thicker wall rear bumper now no longer can hold the sewer hose, so I had to add a PVC pipe under the trailer to store the hose in.
In closing, I would not mount anything over 50 lbs. that may have a twisting effect on that stock thin wall bumper - that includes a bike racks or spare tire without replacing the bumper with something more substantial. Otherwise the stress will tear it loose at some point!