Fair to mention the Vixen Motorhome here? Circa 1989. Cd .31 (actual number - not a scale test).
To me, the lesson is (as has been stated before) round the front as much as needed (which isn't that much) and taper the rear as much as you can. Done.
The result? A brick that is more aerodynamic (relative to Cd) than most cars on the road.