Unlike the 1990 and newer engines with multi port injection, the EGR tubes and throttle body should not need cleaning. On these engines, two injectors squirt fuel down the throttle body which ends up self cleaning things.
You might try bumping up the timing a few degrees. Bump it up until you hear a slight ping at low rpm's / heavy throttle. I run mine at 13 degrees advanced and use the cheap gas.
You can also replace the muffler with one that is more free flowing.
Also, check your idle speed. It should be around 550. If it is much faster than that, it is wasting gas and the idle speed controller (ISC) may need to be replaced.
Believe it or not, driving habits can really affect fuel economy. I find that with city driving, getting into the habit of letting off the accelerator and letting the car coast well in advance of stopping at a stop light saves gas. Every time the brakes are applied, all that energy is lost in the form of heat.
Also, avoid warning up the car or prolonged idling. Lots of idling will eat gas mileage up very fast.
I have basically the same car as you (1988 Eldorado 4.5L) and get a consistent 23 city/32 hwy with stock 14" wheels and tires. However, with my cool looking 17" wheels and low profile wide tires, my mileage drops down to 22 city/28 hwy.