Top three gears are the same. Frontal area is about the same, and even if the new one is as much as 4000 pounds heavier, frontal area matters more.
If you've gone from E-350 to E-450 chassis, gearing is about 10% lower. So RPM will be a bit higher at the same cruise speed, but the motorhome might be downshifting out of OD less often for the smaller grades.
Don't understand the second-lane, third-lane stuff. Most of the time we have only one lane going in each direction, and if there are two, it is "keep right except to pass" so there are no issues about trying to keep up with traffic cruising in a higher speed lane.
I've averaged 8.3 MPG over 30,000 miles with an 11'6 tall 102 wide C, cruising 60-65 most of the time unless slowed down by headwinds or getting a good push from a tailwind in a 75 MPH zone, then I might run 70 in the right lane if it is moving that well.
