There is a MB Service Bullitin on this "slumping issue".
I won't lie to you, these ABC cars are a PITA to diagnose.
The first step is to completely flush the 14L of pentosin the
ABC system uses as well as change the filters. They will also
need to preform the "RODEO" program and make the car cycle
all of it's valves to self flush and work the struts.
Then the car must be parked level and checked overnight
If this occurs, the level control valve blocks will need to be
replaced because of intenal bypassing or leaking.
If your dealer has been in contact with the MB help line,
they should allready be aware of these things.