I'm surprised you haven't an answer yet from the forum. This question has come up several times before. I would suggest two things.
One, do a search on the forum and see what comes up.
Two, call A/S direct and ask them the question. The dealer should know in most cases but the factory knows in all cases. I have called them several times and always gotten good info. Sometimes it takes a while for them to call back but they always do.
Just as a reference point, my 83 280 Turbo Diesel manual sez 2000 lbs. I have been under my unit and the limiting factor seems to be the frame construction, can't be made stronger because my grey and black water tanks are mounted just in front of the rear bumber.
Towing capacity is mostly related to structural strength, not engine size. Next comes the size of the tower and the towee.
Just a thought.
82 280 Turbo Diesel
A/S # 1A9TDAM20CJ009822
VIN # 1GBKP37W4C3300759
Rear bath, twins