I thought you ment any 5th wheel, You didn't state you were looking at the titanium.
I had a good look at one of them rigs last summer and just by design alone I think it could give any truck a 10% saveings in fuel mileage. They are very aerodynamically designed on the front end from top to bottom and especially side to side. They are shaped like a inverted V on the front from side to side, very aerodynamic.
Ron