The L5P motor seams to be one of the most reliable Duramax's I have owned. They are work horses don't treat it like a 1/2 ton work it will last and get the job done fairly easy. Some guys buy these trucks as a daily driver and never work them and may not know how to maintain them.
the 2020 and early 21 trucks had a DEF tank heater issue that was fixed with a reflash and possible a tank heater and harness ( the connector burned up 2 pins at the tank ) . Mine has been fine now 2 winters since the repair by GM.
DEF , if your not working the truck your going to have a system with possible problems of clogged DEF injector.
2015 42' Redwood RL38 Morryde IS , disk brakes, 1920W of solar with Victron everything,Battleborn, 2020 GMC DRW 3500HD Hensley BD5 air ride hitch