It would be interesting to see some studies of RV aerodynamics but as every RV is some sort of compromise, after a certain point improved aerodynamics will likely be at the expense of something else.
My present 5th wheel has very good aerodynamics for a 5er but the trade off is reduced bedroom closet space and basement size.
I'm thinking that contouring the 8' X 8" rear end enough to make a real difference in aerodynamics could make an RV quite a bit longer with a decrease in functionality. When it comes to efficient use of space, nothing is more functional than a box.