Sounds like the OP's mind is made up, but just adding a data point for those interested in this in the future. My 2020 5.3L averages around 20MPG on the highways and 17 in the city. This is with auto start/stop manually disabled (every frickin time I start my truck) and a CAI cold air intake. Everything else stock. I have a slight lead foot. I'll be pulling a 22' center console boat (roughly 4K lbs) soon and I'll report in the gas mileage towing that to the beach and back (2-2.5 hours).
edit: the 20MPG is what I see normally (day to day doing about 45 miles and mix of city/highway). My best 400 mile MPG was 21.9 which was done on a trip from Orlando, FL to Durham, NC.
Also, I'd never consider diesel due to additional cost from DEF and the fuel itself. Maintenance, other than oil changes, I have done via the dealer or a local shop I know who has serviced other vehicles of mine. Diesel has always a more expensive option for shops to do maintenance on.
In the end, gas 5.3 was the best option for me and pairing that with the 10spd auto makes it a no-brainer in my book for those who don't want to deal with the hassle of diesel or premium in a 6.2.