We have a 20 foot enclosed slant nose trailer that we pull our Nissan 350z roadster show car in. We have a weight distributing hitch, sway control, the 3.73 rear end, and an electric brake. We have traveled from Atlanta to St Louis, Orlando, Myrtle Beach, Memphis, and Fontana NC (Tail of the Dragon).
The sway control really helps as does the slant nose. The electric brake having a lever to apply additional brake really helped going down mountains.
When the trailer is empty it does tend to bounce around a bit more. It is a much more stable set up with the car in the trailer.
The only time we had issues was going over the mountains to get to Fontana. The truckers were having problems too so it was the grade that was the problem. When we topped one mountain crest we were only able to do 25 mph even though we started the mountain going 60. The truckers were going even slower.
Our setup is towing right under 7k lbs and we tow fine. The mileage is probably around 11 or 12 especially since we have mountains/hills as soon as we leave Atlanta.