I have one that you can see in my signature. I have ran it for years because I got it free from my father who went to a Motor home. I took it off for 6 month during this year to do some repairs on it. The only thing I noticed different was the bugs on the front of the trailer when it was off, as far as gas mileage you will not see anything worth the cost of the wing. I get the same mileage with or without it. They are not wide enough or tall enough or close enough to the trailer to work efficiently like the do on tractor trailers.
just my 2 cents worth.