You should have the trouble codes pulled forst. Even if there is not a check light on the dash that is lit, there can still be a code present. Any advance Atuo or Auto Zone will do this for free. This will check the elctrical system for faults.
If there are none present you can start to look at the air shocks, distribution blocks and valves as well as the connections and lines for a leak. since the front shocks are faulting togethr. You may look at the problem starting from the front to rear leveling valve and work forward fromt there. It is simply a visual isnepction. you cannot do much more without special tools that only a dealer or air suspension specialist will have. I am giving you a link to one of the best places on the internet for air suspensions on Mercedes. They are great for information and parts.