I have a 383 in my 1990 Suburban. I can pick up at least 2 mpg on the highway by staying in the right lane and setting the cruise control to 60, instead of 70 or more in the fast lane. We're talking an increase from 14 to 16-17 mpg. Your '86 big block probably doesn't have overdrive, but you still should be able to go from 9 or 10 mpg to 11 or 12.
Gearing and load are factors, but on level ground I think the major issue is that these trucks have the aerodynamics of a brick; wind resistance increases as the square of your speed. Slow down and save.