This is from My notes not a comment by me.
The below site is not directly for your year and model. Go to the home page and do a search for your year and model.
Curt Trailer Hitch for Mercedes-Benz 300D 1984 - 11614
w123 300TD hitch question
I also have a TD and have searched for a hitch in the US with no luck. I have found new hitches from a company in England, and Bosal in Germany. Oris and (I think) Bosal were OEM suppliers for Mercedes.
I have seen a few used ones on eBay Germany, and a new one manufactured by Starr. None of them have detachable balls. The TD hitches are unique and can not interchange with the sedan or coupe. I have seen two different capacities of 1500 & 1900 kilos with a tongue weight of 75 kilos.
From what I have gathered, the electrical is also somewhat complicated and you are better off to get the correct wiring set for your car.
Here is a link to one for sale on the Dutch W123 site. He is asking 100 Euros + shipping & handling
My wagon has a Da'Lan class II, square tube receiver hitch which was on the car when I bought it. I believe that the unit bolts under the bumper brackets and to the subframe stampings on each side of the fuel tank.