The '15s dropped 700lb by going to an aluminum bed so the tow ratings went up. Engine is the same. Engine tq/hp gets a bump for '18.
For your use I'd search for two things
1: HD towing package which should get you 3.73 gears vs 3.55 (mine has 3.55 but with 33" vs 32" stock tire so equivalent 3.43 diff)
2: HD payload package which gains you stiffer frame, higher payload, and higher capacity axles (easily spotted by 7 bolt wheels vs 6)
Also note that the advertised towing capacity is the maximum possible and takes a very specifically configured truck.
You need to look at the door sticker to see the actual towing capacity for the truck as configured.
Did you notice any loss in power or mpg's with the upgrade to 33's?