This is from D.M.Waters' research,which appears in Hucho's section on RV travel trailers.
Waters tested the drag of a rectangular box with progressive rounding of all edges.
The drag is a function of the size of the radii,expressed as a percentage of the square-root of the frontal area.
For simplicity,I've used a rectangle of equal width/height.
Three views are represented: side elevation at bottom,plan-view above it,and front elevation to the lower left.
You may see a similarity to Airstream type bodies.
From Waters' curve,the drag minimum occurs when the radii are equal to 39% of the square root of the frontal area.(lower than a cylinder with two convex hemisphere's tacked on each face).
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Here you see how E.M'o'ller softened the original Volkswagen delivery van of 1951 to shave the coefficient of aerodynamic drag from Cd 0.76,to Cd 0.42.
This is a 'guess' as to what M'o'llers model may have looked like.The data is very limited
Wolf Heinrich Hucho experienced similar results during the wind tunnel development of the Volkswagen Vanagon in 1969.