The tailgate latch may stick causing a drain on the battery while the vehicle is parked.
Car Problem Reports
Mercedes-Benz ML63 AMG Rear Tailgate Latches May Hang up and Cause Battery Drain
Mercedes-Benz ML63 AMG Problem
Visitor, 2007 Mercedes-Benz ML63 AMG, 34,963 mi
I reported tailgate problem when I took it in for dead battery. They reported that dead battery and that fixed the tailgate. The tailgate is worse now.