Air Tabs

I've had them on two coaches and would put them on a third. They do help keep the toad and rear of the coach cleaner and they help the stability on the highway and in windy conditions. If they improve mileage, I can't say since I don't keep track of my fuel mileage. I bought them for the handling improvement, not for any slight increase in mileage they may offer. For the cost of lees than a tank of fuel, I felt they are worth the cost.
Personally I have seen them on quite a few vehicles. Most people are not looking for them and since they can be painted to match the color of the rig, they go unnoticed.