I agree that forcing the transmission to shift into 6th will reduce the RPM's by 150 or so.
But reducing the RPM's does not improve the fuel mileage performance in fact, in my experience, it reduces the mileage performance.
I have a ScanGauge which is manufactured by Linear Logic. This instrument, which plugs into the trucks computer, provides many different readings on the status of the engine.
One of the gauge choices is to enable a real time fuel economy readout which I have used to validate the deterioration in mileage when shifting to 6th gear.
My experience has been that while towing on a perfectly level road at 55 mph and using cruise control in tow haul mode I will note the real time fuel economy. I will then force an upshift to 6th by disengaging the tow haul mode while continuing to use cruise control.
Immediately, the RPM's will decrease, but the real time fuel economy will decrease from what was being achieved in 5th gear.
Thus the benefit of a decrease in RPM's in this instance is being more than offset by an increase in the amount of throttle that is required to maintain the constant speed of 55 MPH in a higher gear.
Using cruise control ensures that it is a computer and not a foot movement that is changing the amount of throttle.
So in my experience, RPM's do not tell the whole story.