I know how bad gas mileage feels, so I feel your pain
What RPMs are you running at (this is such bad mileage, I think it could be related to the tranny actually).
Do you run the reccomended octane?
Also, the basics are air filter (may need to be replaced), fuel system cleaning (I highly reccomend Techtron, made by Chevron), spark plug fouling (NGK is the name you want to use for Nissan), etc, etc.