I had a 21 High Country 3.0l diesel until a month ago. The reason the tow rating was lower until the 22s came out was due to the engine cooling and the fact the DPF is under the hood and not under the truck like the 2500/3500s. The drive train was great but the truck was a POS. All the modern electronics was the issue. The BCM did a meltdown and caused the truck to be completely useless (no dash lights/no start/emergency brakes locked). That may have been the last GM I will ever own. Took almost a month to fix. The dealer had the new BCM the next day but couldn't get anything in the truck to work. A GM engineer had to come to the dealer and do a computer restart like is done in the factory to get it to work again.
We moved from Colorado to Casper Wyoming last Jan/Feb. Towing a 20ft enclosed trailer (about 5000lbs loaded) with the truck I got between 20 and 23mpg depending on the wind. Empty on the freeway I could get 30+mpg.

Jay