Check all filters, or just replace them if they are of unknown age. Check for fuel leaks as well. Wouldn't hurt to check for dragging brakes - jack up each tire, the wheel should spin very easily with almost no drag. Increase the tire pressure to 32-34psi if you can, the low MB recommended pressures are more for comfort than MPG. But yes, 22.8 seems low. Make sure you are doing the math properly, you'd be amazed at the number of people who don't do this correctly:
1) Fill up completely
2) Reset trip odometer
3) Drive until the reserve light comes on
4) Fill up completely
5) Divide miles traveled (trip odo) by number of gallons used in step #4
6) Repeat starting at step #2
You get much less accurate numbers if you fill up before the tank is near empty.
'Italian tuneup' means to drive it like you stole it - lots of full-throttle runs to the redline, uphill if possible.
This is not hard on the engine, actually it's GOOD for it, as it burns out carbon deposits etc and often makes things run better afterwards. You should get at least one or two WOT runs to 60-70mph every tank to help keep things carbon-free. If you see lots of black smoke (at first), it's working as intended, and the smoke should get less and less as you drive it harder.
