IMO Unless it is tied into the frames it is doomed for failure. The OEM bumpers are not designed to handle any weight what-so-ever.
It also may be a DOT SAFETY item if the BRAKE LIGHTS are being impaired.
I cringe even having my SPARE MOUNTED on my fifth wheel trailer...
This google link sums it up pretty good...
BUMPER LINKHow much weight are you adding to the rear of the trailer. Is this going to effect handling of the trailer as well...
Another idea would the attached frame that has two swivel wheels under it.
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Any of these additions will require a beefed-up rear bumper however.
Just saying
Roy Ken