My bro in law has the same as your dad. Same issues constantly kept me away from diesel.
had this same truck, sans 8 spd, I’d get 19-20mpg on the highway…
around town is a different story though…that Hemi has a lot of snot.
8 spd with cylinder shut off and if I catch all the around town lights right I can get mid 20s on the dial. That all comes to an end when I have to stop and then the 8cyl kicks on and it drops down. I have been really monitoring my driving habits and have done 75-25 City/Highway and I am averaging 16.7