Sorry to hear that!
This is exactly the stuff that Mercedes should be running to Resolve, instead it sounds as if the dealer is Hiding! Obviously saying that "all of them do that" is not good enough! Fix it right or replace the damn thing. This is poor Customer Service and poor Standards!
I am sure that ML55's are not exactly a dime a dozen, the very least you deserve is exactly what you paid for, a NEW (defect free) Vehicle.
Go to the top and let them have an hear full. Get in touch with the highest person involved with resolving this and then tell them how you feel, consider this your legitimate chance to give someone a real Ass Chewing! This is your right as a Customer! Maybe it will light a fire under someone's behind to resolve this.