MAFs (mass air flow sensors) are common issues with the 210s...this may be defective without a CEL yet. Additionally, I have had to replace multiple O2 sensors on my 1999/2001 E320s.

You may also be experiencing the 10-20% mileage decrease as a result of new 10% ethanol content in gasoline.

Wait a little longer or scan for codes (not sure if anything will come up)...don't throw the parts mentioned above at the problem since they are costly and may not be effective.