Talk to ArcticSteve about his Maxima. He's got a newer one, and can probably pass on to you what his lifetime fuel economy average is.
It IS a big heavy sedan with a great big honking V6. Still, 15L/100kms is worse than my Grand Caravan, also with a great big honking V6. If it was brand new, I would attribute the horrible mileage to the engine not yet being broken in.