So far I haven't had a real long run,only about 200 miles, to see any MPG results, I really think it will not be a large amount of difference. We ran Vortex generators on some of our trailers when I was working and saw some amount of fuel savings and over the road stats that made them worth while in the very large amount of miles that we traveled. It also stabilized the trailers in windy and inclement weather,as an added bonus it kept the rear doors cleaner and free of debris and ice.
My findings so far is that it has greatly reduced the push from being passed by trucks and passing them,also the handling in windy conditions has improved,and the rear of the RV is cleaner.
I did this for better stability,not so much for better MPG but any savings would be a plus. So far I am happy with the results.
When I get more miles on them I will return and put those findings up.