Towing Wind Deflector

Anybody have one of those wings you mount on top of your truck to deflect the wind over the trailer when towing? Do they work? Willing to loan it out? I'm making a 1600 mile trip for boy scout summer camp this year, towing an 8.5x18 high top trailer with a '97 Suburban (gas). Last year, I got about 6 mpg, so anything I can do to increase that, would be worth it.
 
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