Sleeker designs can save some fuel but consider that some RVs are a barn door going down the road and others are a slightly curved barn door. :B
It has always seemed to me like getting rid of some items on the roof would be low hanging fruit in terms of aerodynamics.
All those air conditioners and vent lids sticking up into the airstream have to be detrimental. There are a few RVs that put the air conditioner/heat pump inside a cabinet rather than on the roof. The vents, antennas, and other items could also be streamlined.
