The designs you show are thoughtful. The first one should be very stable, it's used on land-speed record cars. The second one I've seen called a catamaran design. That was the first thing I thought of when it sounded like the question was 'what's the perfect single number for all cases everywhen'. The third is my least favored.
It looks like you've been thinking about that mandated hole in the top, with some drainage channels at the back. In some ways it more important than the bottom, being more tightly regulated. What would you think of an everted NACA duct for the leading edge of the opening? Or just adding a half-circle to the rectangle with a plexigalss windscreen. Angle the sides up past the driver's eye level like a F1 race car?
How do you find the NURBs modeling? Is there a tool path to get you to a cutting pattern? I like polygon modelers because you can pick dimensions off even non-contiguous vertexes/vertices.
Here is something to tie it all together: aerohead's example (just implemented as a three-wheeler) in a polygon modeler (just blown up to motor-home size*). It's completely malleable on the X-, Y- and Z-coordinates, to fit your need.
*I should update the graphics since the motor home donor vehicle is gone. Think Tesla Model S drivetrain.