Where did you get the idea it will get 30 mpg?
20 is good for a gas 6 cylinder.  30 is diesel territory.

I've owned the same model since new. I'm able to achieve 26.9 mpg on
long freeway trips at consistent 65 mph; I don't bother checking local
driving for I know it will be lower.
The EPA for this model was 20 and 27 mpg.

I installed Bosch Platinum plugs - pre gapped with two grounding
electrodes. They work fine but so did the original Bosch conventional plugs!

You should know that this model had a recall for its side door airbags
in July 2000. You can call MBUSA to check if this update was done (free)
to your car.

The only thing that I've noticed is that the car's front wheel toe-in
really affects its steering and tire wear. Check the toe-in if you
notice that the steering seems "squirley" at low speeds or that the
tires are wearing excessively.

Otherwise, the E320 is a fine freeway cruiser that will give you a lot
of satisfaction.