I have also heard that the older V8s will average about that for mileage. A 450SL is a pretty heavy car. You might do a little better on a freeway. Country roads a often winding and have hills, slowing for twists and turns and then accelerating on straightaways can do that to mileage. You might have things like, plugs, wires, cap and rotor checked out. I have also heard that if your timing chain is stretched and timing is off a little a new chain can help mileage. Even with all of the above, you probably couldn't expect more than 1 or 2 more mpgs.
1995 E420 - retired
1986 420SEL - retired