You know what I would do? Go for a short ride in the trailer yourself. If it rides quietly down the road, it shouldn't traumatize the new dog. If it creaks, bounces and rattles - well, it might be "scary".
When we used to first trailer a young horse, one of us would ride in the trailer, just to make sure the youngster was calm and not doing stupid stuff. As a rule, the babies all rode quite nicely.
And lastly - dogs are plenty adaptable to a less-than-smooth ride: my late Aussie Ike used to LOVE riding on our SeaDoo. We'd idle out from the shoreline, and he'd look at me as if to say "This thing has a throttle - get ON IT, woman!!" A SeaDoo at 40 mph is loud, bouncy, wet - and he loved every second of it....LOL!