Traded my 2018 2500 Duramax due to a battery constantly discharging.
Dealer could not determine how to fix it.
It started at about 4000 miles. After having it towed from my driveway the third time, it had to go.
I loved that truck but had no trust it would start when I turned the key.
I have been researching and read somewhere that sometimes the Body Control Module, BCM, does not shut-down and it can drain the battery in 2-3 days.
Not sure if this was my problem, but may be worth asking for yours. Good luck!