Looking for help/advice on truck purchase

[Old Driver]

I just bought a 2010 peterbilt 386 with a cm871 in it. 520,000 miles. 3.55 rears. 13 speed. Had it demandated immediately. Pull fuel tanker so empty half the time. Goverened at 65.
With cold Minnesota weather I am floating around 6.9 miles per gallon. Sometimes with better driving can get 7.5 mpg.

I don't sleep in the truck much so need to have a big sleeper, fuel economy and weight is very important to me when I was looking for a different truck than the 2007 t600 I used. That truck was also demandated and averages 7mpg pulling tanker.