I just did a run to Idaho Falls last weekend and got 19mpg @ 83 mph.
I also got a 16" crack in my windshield when some asshat merged back into my lane (right lane) about about 10 ft in front of me in a stretch of fresh gravel. grrrrr......
Oh well, Safelite is going to pay my deductable, so no money out of pocket, just time and aggravation.
I usually run 85 octane as well, even towing. The one time I towed with premium I was climbing the grade to Strawberry Res. with about 8200# total trailer and gear. At the last short steep section at the top, the truck temp climbed a bit and the computer said it was reducing power output. The outside temp was about 95*. I've done this before with reg. gas and had no issues. I'm wondering if the higher octane allowed the cpu to lean out the mix more and it ran a little bit hotter.
On one trip back from the swell, strong tailwind and 65 mph speed limit and I got 22 mpg !
If you are towing a travel trailer or something else with a large frontal area, speed kills your mpg.
My trailer @ 65mph and no headwind would yield about 9mpg. Toss in a headwind and it drops to 7.