If you can get a hold of someone that tows RV trailers for a living, ie delivering them. THEY WILL tell you that there is some difference in mpg with different trailer designs, along with sidewalls. THe smooth fiberglass walls will net some additional MPGs. A non front slide 5w with smooth walls, the best of std style rv's. Airstreams get the best of the pull behinds.
Now you are into as someone noted. how much improvement for mpgs?!?!?!? yes, some rigs with rounded fronts tow easier generally speaking also.
The thing to look at that a log of semi's are doing, is look at the rear of the trailer. SOme designs are better than others for airflow improvement, which at the end of the day, should allow for an easier tow.
Marty