Turning NASA's 'Shoebox' into a Slipper
 

I picked up where NASA left off with their 'Shoebox' van streamlining and took it to completion.
Starting from their fully-boat-tailed Cd 0.238 van I:
*added body side camber (Buchheim,Deutenbach,and L'u'ckoff)
*added upper edge radii (Hucho)
*added roof camber (Buchheim,Leie,and L'u'ckoff)
*added wheel skirts (Heltemes,Tremulis,Cogotti,Garrone,and Masoero)
*added MOON-esque wheel covers (Shinella,Tsukada,Sakagami,Arai,and Takahara)
*and full wheel fairings (Tamai),allowing 50% of Tamai's factor allowing for wider tires (SWAG).
http://i1271.photobucket.com/albums/...titled1_14.jpg
http://i1271.photobucket.com/albums/...titled2_14.jpg
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We'd have to add back in some features drag associated with a working cooling system,wipers,and mirrors,but that's not a lot of fruit to put back on the tree considering the GM Precept concept car.
PS these are just estimates but should fall close to the tree.

Now if somebody would make an RV like that with a I4 Cummins for power. Bed in the tail like a V-berth in the front of a boat. Make the interior 6 1/2' tall in the center and make it 7' wide. What would the frontal area be about for that?
I bet it could get 30 mpg

or maybe better yet this motor
http://cumminshub.com/2.8l-cummins.html

Angle back the windshield a bit too as some newer buses are doing.

You've effectively re-contoured the entire body. Why not start with a
Bowlus Road Chief? They are being cloned today.

http://smalltrailerenthusiast.com/wp...2013/11/62.jpg

The 'tow vehicle' could be a Tesla drive train/Segway hybrid that would drive, steer and brake—with a trailer hitch. It could have a head like that BB-8 droid in Star Wars Episode Seven.

frontal area
 

Frontal area would be about 38.22 sq-ft for the cabin,plus axle and tires,'n anything else.
Seems like a modern TDI engine would have no trouble with 30-mpg,since Jettas are pulling Sport Airstreams at 32-mpg.