I don't pull a TT, so you can tell me if this is a bonehead idea . . . but why not just transport the bikes inside the trailer? You won't use them until you reach your destination anyway, will you?
Wouldn't having them inside, specifically close to being over the axles, give them a smoother ride than bouncing all over the place hung on a rack on the very end of a trailer?
Plus, it would keep them dry, and free from road grime, debris, and rock chips, right?
Just a thought -- but a thought from 'an outsider looking in' -- might not make sense, but it seems logical to me.