I might be wrong, but I didn't think the MB's had air. You should have a hydraulic suspension. Check the suspension fluid in the resevoir under the hood, this fluid should be changed periodically to maintain the suspension system.
From the description of the ride, it sounds like you need a pair of nitrogen cells (accumulators). Bed cells can give a log wagon ride.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.