The only thing that can catastrophically cause your gas mileage to drop from about 20 mpg to about 8 mpg is your O2 sensor.
This is the characteristic symptom of a bad O2 sensor.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".