MPG and RPM Question for Chevy truck-New RVers

We have a Suburban with same engine and transmission, but 3.73 gears. I tow at 70mph and get 8mpg. Sometimes a little less, sometimes a little more. I use tow/haul mode on the transmission and I like to use M5 to limit to gear to 5th. I cannot hold 6th gear very long and ends up downshifting to 5th, or 4th when it loses speed. You should use less fuel in 5th than 6th when towing.

That truck can turn 3000rpm all day long towing that trailer.