The W210 transfer case is different from the one on the M class, although they both use ATF.

There is NO drain lug on the W210 transfer case but a filler plug that is hidden behind a vibration damper (the counter weight referenced here). The damper is hard to remove (top nut).

The most probable cause of leak is between the transfer case and the transmission. There is no gasket in between.

Dealer will only do two things

1) gladly replace the whole transfer case at over $6k.
2) reluctantly reseal the transfer case for about $1k.

I bet the leak is very small. So just add some ATF once a yer or every two years.

The capacity is about 1 liter but Mercedes quote different capacity in Owners Manual, WIS, etc. You fill until fluid comes out of the filler plug (allen).

There should be a thread on benzworld by me and OliverK on this.