Yes, the skirts you see under the tractor trailers supposedly result in 7% or so in fuel savings. For someone driving hundreds of thousands of miles for a living, this could add up.
http://freightwing.com/I'm no aerodynamic engineer, but I doubt the rear flap on class As have much of an effect one way or the other. The idea behind the skirt is to reduce the turbulence behind the tractor and under the trailer. The rear flap on motorhomes is toward the back and has little affect on the aerodynamics of the motorhome that precedes it.
