Horrible fuel mileage

New to the Duramax power plant, lots of experience with all years of the 5.9 Cummins. I just bought an 07 classic and I’m currently getting 11-12mpg which seems to be wildly low for a diesel. The truck has 33x11.50’s, FASS, a banks tune (I think) and original calipers that are probably dragging. I just cleaned the banks oiled air filter which has seemed to help a bit…what else could be causing this poor mileage?

Thanks in advance!