18.6 sounds about right. Regardless of cylinder count, you're hauling around 4,200 pounds and that takes a certain amount of power. The weather has been nice lately, which improves it. I'd expect lower in the summer with 100*F temperatures and lower in the winter with colder starts.
That's right at what our Acura MDX gets in town. It has fewer cylinders (6) but it weighs more (4,500 pounds).
Mom had a '92 Crown Vic with the 4.6L, and she got upper teens in town as well. It'd get 24-26 mpg on the highway.