I've had two DieHard Gold AGM batteries die in my 300. I even got an adjustable regulator to charge at the needed 14.5v in cold weather. Both times the AGM batteries had a cell die. They would charge to ~12.8v then fall down to ~10.6v within an hour resting and both failed load test at ~8.6V with 450amps load.
I've since switched to an ACDelco Advantage flooded cell battery last month and its been resting to 12.8V overnight consistently. I wanted a genuine Mercedes battery, but they only have AGM now.