In certain fluid dynamics flow regimes, turning laminar flow into turbulent flow reduces drag......that's what those things are supposed to do...they are vortex generators.........whether they provide any meaningful benefit to gas mileage, etc...who knows.
Please let us know if your van is getting 20mpg
. If things like this worked for improving gas mileage the car manufacturers would be all over them to meet the CAFE fuel economy requirements.
This is the same reason that golf balls have dimples..to break the boundary layer air flow around the ball into turbulence. If you take 50 balls that are smooth and 50 balls that are dimpled and drive all of them you will see a statistically significant increase in distance in the dimpled balls...*assuming that you are not a hack golfer.