I towed a 20 foot enclosed car trailer, 8 feet high and weighing just under 5,000 lbs with a 1998 ML320. It did fine, although it really had to work on the hills to maintain highway speed. I moved up to the ML430 so that it would be easier to maintain good speeds (70-75 mph) up hill.

Mileage on the ML320 towing was about 10 mpg. With my big square box, the wind resistance really reduces the mileage as speed increases. With a travel trailer, if it has an aerodynamic front, the mileage should be a bit better, but I wouldn't expect much more than about 12 mpg with 4,000 lbs.

Good luck!