Any trailer or trailered vehicle (including 5th wheel type) has a slight side to side motion that's very low frequency (1 to 1-1/2 times per second) that fatigues the steel from which the racks are manufactured.
The fatigue first happens at a molecular level, but eventually (several thousands of cycles later) a visible crack will appear. Once the crack has formed, it will enlarge, finally to the point where the rack drops off the vehicle and the rack (and bikes) impact the roadway.
This is also a dangerous situation for whoever may be following the trailer on the highway.
Hollywood Racks have seen 100+ incidents where the consumers/end user has not taken this advice and suffered disastrous results.
They've even had phone calls from consumers who said they read that advice (do not use on a trailer) and ignored it, only to discover a tragic end result.
So . . . don't do it. If you insist on doing it regardless of the above, please use some other companies’ rack.
**Information from Hollywood Racks