Welcome to the forum.
Is 6500# the real weight, or the brochure weight? Most TT's weigh 300-500# more than the advirtised weight due to the "optional" equipment that's really standard. And you'll have 700-1000# of gear once you're ready to go.
What's the GCVW on the Mercedes? And the payload? I have a feeling that it'll be way too much trailer for the GL350.
What's the wheelbase on the Mercedes? Even with a good hitch I have a feeling it'll be a case of the tail wagging the dog.
And don't listen to the salesman when he says "Sure, you can tow that." All he wants to do is make a sale and has no legal requirement to make sure you're safe.
