MPG and RPM Question for Chevy truck-New RVers

I get 11-12 with the 5.3, 3.42, 6 speed driving with cruise at 68 most of the time. I've seen a low tank of 8mpg when I ran into a 40mph headwind for 4+ hours. But I've got a narrow trailer so I get a bit better mpg than most, it's really all about the exposed area outside the footprint of the truck and the speed, going 72 will drop me down to 9.5-10 mpg, definitely not worth the few extra mph.