I saw your thread at DMaxforum:
https://www.duramaxforum.com/threads/for-those-towing-heavy-with-2020-and-up.1026307/post-12493998
This particular post was the only guy that was pulling a heavy 5th wheel (tall) with a GCW of (28.5k) and he is using about 1/2 quart/1k miles. I have a GCW of right at 30k and pull a tall (13') 5th wheel. It takes serious power to pull heavy and tall, especially into the wind. He should try lowering the sustained EGTs by pulling his 5th wheel in 8th or 5th if he has a six speed.
Another poster said, "If that is true, it would make me think turbo seal is bad. He would be running higher boost in 10th vs 8th". This is flat wrong, you don't run higher boost when pulling in 10th, you run higher boost pulling in 8th, but not by more than 1 psi. There simply is no substantial difference.
Most of the replies in your post don't pull heavy and tall. It's like comparing apples to oranges. I used to pull an Airstream and got 14 MPG. When I pull my 5th wheel I get 9.5 if there isn't a headwind. Pulling into a headwind sucks fuel, I've averages as low as 7 MPG at 55 MPH.
@Doug Hillary used to measure work by how much fuel an engine consumed, I think it fairly applies here as well. If we are going to compare who uses oil and who doesn't, their averages fuel consumption should be close to mine.