15 mpg isn't good for a 6-cyl W124...unless you tow a camper uphill on a regular basis...
Yes, there are a number of factors that can impair fuel efficiency.
My first question is how is your engine's performance? Does it idle and run smoothly?
Fuel injectors may get clogged over time, and that will have a drastic effect on fuel consumption. Do you smell any gas fumes when on the road? Check for fuel line leaks as well. I had a fuel line give up a few months ago. Didn't leave any puddles, but gas mileage was horrendous!
A bad O2 sensor would set off a code. Check for codes first.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car