I've got a 'bug deflector' that came installed on mine. Don't know if it help mpg but it does seem to help with frontal air flow. And the lower half of the windshield is usually clear of rainwater at highway speed, probably a side effect of the wind flow. Haven't been in bug season yet.
I've seen studies of the rear mounted 'spoilers' on other kinds of rigs. They generally do not improve mpg, as they create a larger 'turbulence pocket' that is an aerodynamic drag.
get in--buckle up--hang on
"soothing agricultural implement/personal servant/Walter Mitty multidimensional access utility device conveniently travels on pavement when necessary and often keeps me warm/cool/dry/soothed as needed."