Fuel mileage still sucks

[Lead_sled]

So, I’m seeking suggestions now

I’ve done a tune up, brand new CAC, and new rubber CAC silicone hoses.

My engine runs strong and fine. I’ve tried changing driving habits. Also considered trailer against the wind. I pull a chassis trailer that has removable containers. When I have an empty box, I weigh 36-37k. When full, I weigh 78-80k. Not sure of the weight without a box. Between all 3 options, my fuel mileage barely changes. Below are the numbers. What can you guys suggest me checking?

80k - truck, trailer, full box - 4.3 to 4.5 mpg
37k - truck, trailer, empty box - 4.75 mpg
? - truck, trailer, no box - 5 mpg

I keep looking for fuel on the ground, but none. No idling either. I have a generator with portable inside the cab.

All hand calculated. But even the onboard computer only shows between 4.8 and 5.2 on its readout.