Your mileage is on the low side. I too have an 88 560SL and get about 17 mpg in moderate speed country road and town driving. All-town expect about 15 mpg. I have seen near 20 mpg on a steady speed 200 mile highway trip.

My car is driven daily and I maintain it well. Fresh plugs, fresh PREMIUM-grade fuel and elimination of vacuum leaks if any will make a big difference. They are great cars. Any minor penalty in fuel mileage is amply repaid in the joy of driving.