To try to answer your question, I have a 2021 GMC with the 3.0 diesel. It does not have the max trailer package.
I tow a 2017 Winnie Mini 2108FBS on fairly long tours (2,000-3,000 miles) on a regular basis. The TT weighs 3800 dry, and about 4500-4700 loaded.
Staying at a conservative 65mph while towing, I achieve about 15.5 to 16 mpg of diesel, and it consumes a gallon of DEF every 350 miles. The truck tows effortlessly, it downshifts on hills, but typically pulls at around 1600 RPM all day long. In the year I have owned it, I have towed about 9,000 miles.
Without the trailer, I typically achieve 25-28 MPG and about 900-1000 miles per gallon of DEF. It is awesome for all sorts of trips, short or long. Plenty of power/torque to get moving, and it chugs all day long at 75-80 mph on the interstate.
I am spoiled by this truck!