Search the forum for "Mercedes GL" and you will find lots of information on towing with your GL. There are several of us on the forum with the combination of the GL and an Airstream. My 2011 Diesel GL 350 has proven to be an excellent tow vehicle. You will also find that Mercedes knows almost nothing about towing - any advice from your dealer is likely to be wrong / misleading - that's at least what many of us have found.
I'd also recommend you head over to the Mercedes owners forum, and read the posts about towing with the GL - GL Class (X164) - MBWorld.org Forums
Most important is to understand whether you need your hitch receiver reinforced before you attempt to tow an Airsream with your truck. With this vehicle, you need to be thinking about tongue weight as much as you are about trailer weight.