LOTS of info here at Ecomodder, mostly in the aerodynamics forum.
One thread there
(You could add "ennored" to narrow any search you do, most of my posts have something to do with Motorhome or trailer aerodynamics.)
The Vixen, mentioned here many times, is to my knowledge/research, the most RV aerodynamic (lowest Cd) ever built/documented. It's Cd was .29 for a real vehicle, in a wind tunnel. (It also kept the frontal area as small as possible, the roof popped up on the original model, to lower total drag.)
Bill Collins, the Vixen's creator summed up the aerodynamics development very simply. He rounded the front end until the drag got no better, then he tapered in the rear all he could.
A custom build could follow his method. Round the front corners (4% X width radius has been shown to work), then boattail the rear. You'd be under .3 Cd, about 50% of the drag of a "normal" motorhome.
(I MAY have detailed Vixen info soon, just walked by a guy's desk the other day with some Vixen pictures, and his Vixen business card, yes, he worked there. I think he owns one.)