Terrible fuel mileage 97 9300 with cat....

Hello,

4.7 MPG!!!!!!??????
Everyone is always super helpful on here when I have a technical question concerning my truck. However, the truck is running great right now. No issues to speak of except one huge issue with fuel mileage that has me thinking it may be time to give up on this old truck...

The truck is a 97 INT 9300 set forward axle flat top with a Cat 3406e and 13 speed high torque eaton tranny. When I was working the truck in mud and short runs I never thought too much about its terrible fuel economy. However, for the last 2 months I have been running from Indianapolis to st. Louis and back every day. All interstate pulling a van with aero skirts. I'm not driving over 67 at any point of the trip but I am averaging between 4.7 and 5.1 mpg on every trip.... 30k one way and about 17k coming back.... this is terrible. Running 11r22.5 tires. Can't go to low pro as the tanks would likely drag over tracks. 67 mph running about 1600 rpm. Have thought about lowering the rear end ratio to like 3.35 or something close to get revs down. Not a real powerful tune on it. It basically runs out of pull at 1600rpm which seems odd as I hear of guys running up to 1800 with these motors. I would never do that though just simply this motor doesn't pull beyond 1600. It was overhauled 400k ago. Tranny rebuilt last year. Help me figure out a way to get this old girls mpg up so I can justify keeping it.

My thoughts have wondered into these areas so far.

Reduce gear ratio

Add aero wing to the flat top

Retune the 2 month old ecm

Add balance masters to wheels (reduce vibration and rolling resistance)

Maybe convert to 6x2 with locking rear?

What do you guys think would work??