Hey Thom, I can't give you a figure on magnitude or anything but I can say that a radius on roof to wall will make the rig way more "forgiving" of any aerodynamic upsets such as cross winds and design issues where pressure variations are in close proximity to each-other.
Took the liberty of drawing a rough shape of what would be the best thing to do for a 5th wheel. It is a 25 footer with 7.75 foot ceiling max and 6.2 foot in the very back.
Full Size Version of Pic Here
I can whip up a drawing like this pretty quick, lemme know if you want to see it changed somehow and I'll do it.