This is constantly debated and a controversial subject.
Bottom line, your trailer has an ultralight chassis.
The bumper is made with thin metal and poor welding.
The only safe way is to havea receiver welded and braced to the frame.
But this modification may jeopardize the chassis/frame warranty if the mod causes damage.
Personally, my bikes are too valuable to put them someplace I can't see them until I see them and rack bouncing down the road in my rear view mirror.
I always recommend roof racks, tongue-mounted racks, front bumper racks, inside the trailer, inside the truck bed.
The back of the trailer would be the last place I'd put them.
and Zoe the Wonder Dog(R.I.P.)
2016 PrimeTime TracerAIR 255, pushing an '07 Chevy Avalanche
Equalizer WDH and Prodigy BC