My 560SEL get about the same as Bigshot's, although I do not do much highway driving. Sounds like you checked the car over pretty well, looking for common causes. Tire pressure ok, and transmission shifting right? Are you in an area with lots of hills, stop and go driving too? A/C on hurts too...This could be some of it......But the cars are not econo boxes, all models are luxury cars, not much thought given to conserving fuel. Sorry, not much help here.
1989 560SEL (172k)
1989 325IC (122K)
2004 Suzuki Volusia LE (3500 miles)
2005 Yamaha Road Star (20 K miles)