When fuel mileage doens't change depending on load or speed it is indicative of under gearing.
Exactly why a modern overdrive transmission works so well in these trucks.
I was able to get a 30% improvement with a .70 overdrive ratio in a 700R4/4L60. The 6L80 have even higher ratios with double overdrive, both 5 and 6 are overdrives, with the final ratio in Sixth is 0.67.
The wide overall ratio spread of 6.04:1 allows a “steep” first gear, as well as a very “tall” overdrive top gear for low-rpm highway cruising.
The trend is for higher differential ratios and lower first gear. This tread is nothing new and has progressed since the power-glide days.
Also a another reason to switch to a 4L60. The lower first gear if good for performance.
When in doubt "switch it out".
Rick