So probably a 5.3. That explains your experience. You made a huge jump. Not only did you get the 6.6 but you went to 3/4 ton or better.
It would have been a huge jump in towing performance also from his '12 1500 with the old low hp/torque to the newer 1500 5.3 with 355 hp and 383 torque and a 8 or 10 speed tranny.
Of course anytime one moves from a older low performance 1/2 ton size truck to any new gen 3/4 ton truck with a bigger engine/heavier chassis/more axle load capacity its a big plus.
The wife is looking for a new 2500 GMC 4wd crew cab std bed now to replace her '16 1500 4wd 5.3 355/383 engine with the 6 speed tranny.
She's leaning a bit towards the 6.6 gazzer over the Dmax but hasn't made her final decision.
I'm guiding her towards the gazzer as her truck won't be pulling any of my heavier trailers (14k-16k lbs). I still have my Dodge/Cummins NV5600 tranny for that purpose.