i have posted regarding this before.
what mileage is on your ml? upwards of 60k?
mb claim that the gearbox is filled for life, they do not say who's life.
those in the know recommend that the autobox fluid is changed every 50-60k mls & mb fluid specific to the gearbox type must be used. if its not done the gearbox will overheat and self destruct.
the first sign that the fluid is spent is rising engine temp when towing, followed by the gearbox locking into emergency mode (either 2nd or 4th gear depending on road speed). if these warnings are not heeded your gearbox will go bang soon after.
the ml 270 is rated at 3365kgs max tow so i doubt it has been stressed towing your sterling.
get onto benzworld and you'll find plenty of info regarding gearboxes.