appropriate expressway speed?

I basically will set the cruise control at the speed limit whatever it might be. But when the speed limit is 75 or 80 I'll stick with 70 as the mileage just takes a huge hit. At 22K pounds loaded up just could not keep the truck at 80 on the hills in Utah without burning a lot of extra fuel.

Just did this all the way from Western Oregon to southern Utah with the camper and enclosed trailer.