I wouldn't have a dog in the back with the 5th wheel:
- Airflow changes would disrupt the normal enjoyment of facing the wind for the dog
- Hitch, cord, etc would limit scampering room
- 5th wheel would limit what the dog could do to move around
...but I would and have carried dogs in the back at either city or highway speeds. Typically (not always) used to put the dog in a portable kennel if the trip was more than a few miles of highway driving though.
Dogs LOVE being in the back of the truck on the way to the cabin: an hour or so at 50 -55, then a couple of hours at 5 - 10