This is a hotly debated topic.
Bottom line is that most ultralight trailer bumpers are thin metal, with weak welds. Many have stickers stating that any mods to the bumper, lime adding a bike rack, can void the frame warranty.
Now a non-ultralight trailer can have a much beefier frame and bumper and may norpt have issues.
Most will agree that if a receiver is welded and braced to the frame, putting on a bike rack, would be fine. But again, if any frame issues occur, that mod would possibly void the warranty.
Personally, I have bikes that are not cheap and would NEVER put them on the back of my trailer, where I can't see them.
I carry mine in the trailer or the truck bed.
If I had a 5th wheel, I would go with a pin box mount.
Dan-Retired California Firefighter/EMT
and Zoe the Wonder Dog(R.I.P.)
2016 PrimeTime TracerAIR 255, pushing a 2014 Ford F150 SCREW XTR 4x4 3.5 Ecoboost w/Max Tow Package