I had a "wind deflector". It did not save me a penny in fuel costs. It did do two things for me:
1. Kept most of the junk off the front (nose) of the 5th wheel.
2. Provided a stabilizing air flow in serious crosswinds.
3. Being polished stainless steel it sure looked pretty on top of my black truck!
At a campground a couple once asked me what that thing was on the roof of my truck. I told them it was a "spoiler" like the NASCAR racers use. It keeps the rear duals on the pavement at 110 mph!