I wish I had definite answers for you.
MPG depends a lot on driver habits. Mine is out family truckster. If I get in after my wife the Avg. Begins to rise. As soon as she gets in the driver seat it drops fast.
Here are my indicated MPG.
Towing 11,000 TT @ 62 = 7-8
Empty at 65, 19.5
Empty at 72, 17.5
Empty at 80, 13.5
My wife gets about 12.5 city.
Tows fine, hold out for 4.10 rear axle ratio. It has tall 1st and 2nd gears.
If you expect it to tow 12,000lbs like your cummins, you will be greatly disappointed. No grunting along at 2,000 rpm. You will see 3k, 4k, and even 5k at times. It will downshift, a lot.
Is a gas powered truck right for you? Only you can answer that.