Hi - this could be down to a few things & would always recommend a diagnostic check first to see if any fault codes are stored to indicate any problems. A failing MAF (air mass) sensor can cause poor economy & a diagnostic test routine is available on Mercedes STAR diagnostic tool. Also check/replace the fuel & air filters, a blocked air filter can be a particular problem. Air leaks in the intake system can cause problems as can blocked catalytic convertors. The fuel system is also prone to leaks so check thoroughly for any fuel leaks, although it would likely have been noticed by you already as it would need to be a fairly large leak!
Best regards, Paul