Personally, I would never do it. The whip effect from having the bikes hanging so far behind the rear bumper, which is itself far behind the trailer wheels, is a recipe for disaster. There are very very few bike carriers that are manufactured for use on trailers.
I would think a better option might be putting a roof rack on the truck roof with bike mounts, or there are bike mounts for in front of the 5er's cap, like the Arvika
Fifth-Wheels – Quality Bike Racks for your RV | ARVIKA