under body smoothing - undertrays
When you enclose the under body there are many many things to think about, some of which has been touched upon, like fluid colllection, cooling of parts like transmissions.
I see aerodynamics come into play at about 35 mph. .I bought a bullet butt bus because of aero dynamics. looks cool too. but the trailing side is usually more significant than the front. I will be putting under tray in the engine area, but I have to allow air from the radiator opening to flow around the turbocharger, and exhaust... I will be using temperature sensitive paints and stickers to thermal map the engine bay. air has to be managed around the exhaust, transmission and rear end. There will be a number of openings to allow air out of the under area, I also have to manage temperature around batteries, and skirt mounted condensor coils, and webasto coolant heater.
fenders over the rear tires it is important to keep the fender as tight as possible. smoothing the trailing side of the rear end and suspension. and the trailing side of the front axle / suspension. soothie hub caps on the rear duallies would help some too, look at what is happening on some of the big rigs. tractors and trailers.