The issue is dead weight. While you can tow 4220 (on the ground), anything you hang from the hitch (dead weight) is just like carrying fresh water or cargo, it goes against the GVWR of the vehicle (11,030 lbs total) and not the GCWR (15,250).
If you have 1000 lbs carrying capacity on the Sprinter and you have 500lbs of scooters and hitch, that only leaves you 500 lbs for people, cargo, water.
If you put all that weight on a trailer....only 10% typically transmits to the hitch. With a swivel wheel...I'm not sure what they figure in...but I'm guessing its more than 10%. More like 50%. If the hitch and scooters weigh 600lbs, then 300 may count against cargo. I would verify this with the swivel wheel mfg.
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