Your auxiliary fans should never come on in ordinary circumstances - only in very hot weather and in extreme stop start traffic or started up by the AC switch.. The viscous fan should do most of the cooling in normal operation in stop start traffic.
The viscous fan becomes moot over 25 mile per hour - airflow through the radiator should do all the cooling.
If you are driving fast and the engine temperature is high this indicates a poor water circulation or lack of pressure in the system. Either your thermostat is not working properly, there is a leak in the system somewhere, your water pump is failing and not circulating the coolant fast enough or you have a blockage in the radiator or the cooling passages.
Of course you could have a failing head gasket that is causing heat to get into the cooling system.
You should take your car to a Mercedes independent service agent and get your coolant system pressure tested.