My previous 5th wheel was smaller than my current. It weighed about 2000 lbs less and was about 1 foot shorter in height.
The old trailer did not have an aerodynamic front and had an open frame on the underside. The cross braces, the waste tanks and drain lines were exposed.
The new 5th wheel has the aerodynamic front and the underbelly is covered.
I get about 1-1.5 mpg better mileage with the new trailer. I suspect the underbelly is the major factor in reducing the wind drag.
For most towing which is on level ground wind resistance is the major factor that affects mpg and weight not so much.
Even here in the west the miles climbing hills is very small compared to the somewhat level ground towing miles.