I drove a sprinter with that drive train for 200+k miles. Including a couple of trailers in the mid 3000 lb range. Pulled with out issues. I always felt it could have pulled a 6-7000 lb trailer. Yeah being the engine was a 200 HP 400 lb/ft of torque motor, it pulled well.
I've had the older ML320 version. It's rated to 5000 lbs no matter if you have the small V6 like us, or the turbo 5.5l V8 motor. I believe the new ones are around the amount you state. Again, engine does not matter. Smaller motors a bit slower, less performance than larger powered motors. For me, probably a mid 20' or smaller trailer, that I would max out with gear, water etc in the 6000-6500 lb range, an mainstream or equal that is reasonably aerodynamic would be better than a box. Get something with smooth sides, vs corrugated aluminum. Yes latter is lighter, bit the siding will cause mower use because of wind drag vs the 200-400 lb heavier option with smooth fiberglass walls.
My 02 with similar rigs driven.
Marty
92 Navistar dump truck, 7.3L 7 sp, 4.33 gears with a Detroit no spin
2014 Chevy 1500 Dual cab 4x4
92 Red-e-haul 12K equipment trailer