Personally, I think the chassis battery is fine. Takes and holds a charge fine. It just went dead sitting 2 weeks in storage. I think Mercedes nature of the beast with all the electronics. It was enough charge to start it but barely. I've had similar experiences with an e350 and c300 I previously owned, and a bmw when they sit for 2 weeks. Heck, my 37 year old jeep does it too.
PS it’s super easy to diagnose whether there is a draw or not. Most simple is unhook the battery and wait and if it starts when it wouldn’t have if hooked up, it’s not the battery.
more accurate way is just check with an ammeter in line between the neg battery post and the neg cable (ground). Any more than a handful of micro amps and it’s a problem.
