Please don't let one tankful ruin your spirit.
It takes time and practice to find out what works best for you in any given driving situation.
The SG is a good tool to use, but, it is just that, a tool.
You are the operational factor in this equation.
You are smarter than the truck.
All you need now is practice, which takes time and patience to allow yourself that time.
Many of us started out by going to the cruise unit. It takes the human out of the equation. It also helped me learn I don't have to travel to work at 75+ to be on time.
Welcome to Ecomodder,
I hope (and believe) you will learn a lot here.
Look at the truck threads. They will have the info you want to get your mpg up.
As for mods, you've already done the first two;
Scan Gage, for instant feedback.
Adjusting the nut holding the wheel. (which can be the most difficult)
You may want to try something simple for you first aero-mod.
An aero cap perhaps or a front lower air damn.
Check Aero head and Big Dave, they have lots of good ideas for trucks.
Adiós Amoeba,
Schultz