does it come up to temperature properly?
Ie 90c after a few miles and then stay there?
Also depends where your driving it, you havent said.
If its all around town, stop start 30mph type driving, thats all your going to manage. if your only getting 20mpg on a run down some a-roads sitting at a constant 60ish then theres something wrong with it.
End of the day, its a heavy car, with a large engine, i'm not really sure how much better you were expecting...
I agree with stoaksey, about right really, though these figures you mention are figures I'm getting for a remapped 3.0 so i'd assume a standard one (if it is) would be about 5-10% better!. I've heard good stories about fuel filters being replace, helping performance and mpg, so worth a go at the very least i'd say. Try using high octane fuel for a few fill ups too...V power should clean your engine over 1000 miles or so, you then might see a difference. Don't panic!