my 6.2L only gets 22 mpg highway, but sitting all day at 2750-3100 RPM does not help. thank goodness I have a Fluidampr.
dad's old 84 C30 4spd 2wd chassis cab gets mid to upper teens, but it has a big ol heavy service body. might have 4.56 gears too.
As far as the NV4500, the smoother running 6.5L over the Cummins reduces the issue somewhat, but it can still happen. I have a bucket truck C3500HD that has no 5th.