Sounds as though you have a slow refrigerant leak in the air conditioning system.
Open the hood and look just to the right side of the radiator and you will see the accumulator for the air conditioning system. On the top of the accumulaotor is a sight glass where you can view refrigerant flowing through the system when the air conditioning compressor is running. Under normal operating conditions it should be clear. If there are bubbles, the system is low on refrigerant. If you see a "cloud", the system is nearly empty and needs to be recharged.
Several years ago, my 1995 C280 did the same thing -- kept putting out less cold air. After looking at the sight glass I found a small crack through the glass that was slowly leaking refrigerant until the system was empty. It took about two days to go from great cooling to nothing. The sight glass and accumulator are a complete assembly I replaced the accumulator/sight glass assembly and everything has been fine for the past two years.