[aerohead]

For crosswind stability (anti-roll moment) softening the upper edges the whole length of the trailer will help,as well as reduce drag as you see in the schematic of the bus.(hash marks)
Leading edge softening is a must except for the bottom,which should be flush with the tow vehicle,unless you wanted to treat the trailer bottom as an up-swept diffuser.
The rear vertical edges can be sharp,to maintain a controlled separation line.