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I recently pulled the trigger on a new Sierra. A real looker. GM had a promotion on their 3.0 liter Duramax diesel and I ordered the truck with one paired with a 10 speed Allison tranny. Great boat pulling rig along with being a comfortable trip vehicle. I get about 17 mpg while pulling and my recent 5.3 liter got 10-11 mpg. Not even broke in yet either.
I took the truck to Killeen, TX to see my son Dave and visit other family down there. One day my nephew who is a Army Captain now and an Apache driver took us on a tour of Fort Hood. We got to see his gunship and got some cool pics while there. Had a good time.
Anyway back to the truck. The fuel rating on the Sierra is 26mpg on the highway. This truck did much better than that. The overall average I got was 30.4 during the near 2500 miles I drove. But the amazing thing was the 32.9 mpg it got on flat level interstate in north east Texas. I am impressed with this little diesel. It was also  cost just $900.00 over the price of a 5.3 with a 10 speed. They are fast and smooth riding and so quiet running. Almost can’t hear it. Take a look at one if you are in the market. You’re going to love it like me.

A couple more pics. Check the speed out, 72 mph was a sweet spot.

This is great information! I've been looking at buying a new truck.

Is your truck a 2500 or 3500?
4x4 or 4x2?
Dually or standard?
What year?

It's great to be able to talk with someone who has driven this truck!

Trent the truck is a half ton 4x4 X31. Single axle. It is a 2021. I did not know about the d.e.f.  fluid tank though. A 5 gallon tank lasted about 3500 miles. I had to add some on the trip. 277 horses and 420 ft pounds of torque, same as the GM 6.2 liter v8. The 3 liter Duramax is faster than the 5.3 liter in a 1/8 mile drag race. Of course the 6.2 liter GM smoked it in the same race. It has 420 horses I think.
I forgot to add the diesel makes the 420 at only 1500rpm.

I looked up the specs after I saw your post and saw it was in the 1500 which only has 9100 lb towing capacity but it sure seems like the 3.0 and 10 speed transmission could pull more.

My boat with trailer is any 12000 lbs.
(26ft Stamas Hardtop)

I know the 6.6 duramax has plenty of power and I think in going with the 4 GMC Sierra 3500 crewcab 4x4 dually.

My Searunner weighs about 4500# plus or minus. Pretty easy pull. 12000# rig would need the full size Duramax. But bass boat owners and walleye rigs would be a great choice. A lot less money also.

Sure looks like a great truck Don!