There is a classic aerodynamics experiment where the front of a brick was ground with a smooth parabolic profile and placed in a wind tunnel.
The coefficient of drag was reduced but only by a tiny amount.
When the brick was flipped backward with the parabolic side to the rear a massive improvement in drag was found.
Simply put, the front doesn't matter nearly as much as the back so boxy is fine, you just want to get rid of the turbulent wake.
At the same time, I think aerodynamic drag is proportional to the square of your velocity so going 50mph instead of 70mph will cut your drag in half.
I seem to remember that wind resistance in bicycles starts to become a significant part of the total power required around 15mph. Cars improve that to 30-35mph, Buses are probably in the same ballpark.
At 50mph, wind resistance might be 3x a pretty insignificant amount.
This all points to probably not worth the effort.
But thinking about a rear bedroom in a bus, you don't need full headroom at the very back so a sloped roof might be interesting except it could make climbing on the roof treacherous.
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