there are literally thousands of posts about NOT doing what you want.
the bumper and the welds, on ultra-lite frames, like the Roo has, are thin and will fail, due to the flexing and bouncing the bikes will cause.
there are no shocks on travel trailers, therefore all the bouncing will pull back and down on the bumper.
but since you don't have to worry about the frame's warranty, since it's expired, you can have a welding shop, weld and brace a receiver to the bumper AND the frame.
and buy the beefiest bike rack you can find, that will handle holding the bikes out and stand the physics it'll be subjected to.
can't you put the bikes on a roof rack, where you can see them? makes more sense than putting them where you can't see them until you see them in the rear mirror, bouncing down the road.
a roof rack will be cheaper, safer and usable for other purposes.
and Zoe the Wonder Dog(R.I.P.)
'07 Roo 23SS pushing an '07 Chevy Avalanche
Equalizer WDH and Prodigy BC