I had the same problem just this weekend. I searched for the drain in the torque converter and there was none so I only got 2.5 liters from draining the pan. This is with a 2000 C280, tranny 722.617.

You can get the red tabs from Fastlane, maybe $1.00. The dip stick is ~$30. Make sure the cap is all the way on, it goes on with a click and takes a tug to pull it off. Don't do like I did and not have the cap all the way down and then stick in the red tab. You'll waste a tab because the cap won't stay on. I had the old broken tab so I could see how to pry it off without breaking it and then I was able to reuse it.

I suppose the dealer has some method of forcing out all the old fluid and refilling?

Now I am stuck with 6 liters of MB tranny fluid.

glenmore
1991 300CE
1990 LS400
2000 C280