ISX CM2350 Oil Usage and Fuel Economy - Suggestions?

[Smooth]

Any ideas or suggestions I am all ears.

Recently Purchased
2015 Prostar -
Motor Built in 14' ISX 550HP CM2350 13SP
380,000 miles
Pulling Dry Van - usually 40-45K loads -
Truck Clean, no leaks, runs good, etc.

Clean truck - runs good no fault codes etc.
Run Rotella - Oil Usage 1 Gallon every 650 - 800 miles
After Service was better for first 1500 miles but not good.

Fuel Mileage 4.9 to 5.2 that is weekly over last 30 days so have a short date range of data.