There are lots of people towing Airstreams with the GL350 deisels, lots of torque for that weight and that's a very easy to tow trailer. Will do a fine job according to several reports I've heard from people doing it. Check out the Airstream forums and you should be able to get some first hand reports.
Just make sure the hitch is setup properly, the short wheelbase makes that more important but the Airstream is so stable you don't need to 'go crazy'.
I've been keeping my eyes open for a deisel SUV for when my (aged) Explorer bites it, I think the BWW X5 35D has advantages but some of them are just personal preference and the X5 being much more available on the used market here so I'm not going to push it.