Decreased gas mileage

Has the valve timing been checked? This engine has three timing chains and a very specific procedure for setting the timing. I'm assuming the engine came with heads, cams, and timing chains installed, but builders can make mistakes.

Check for intake leaks, intake hose leaks, check mass air flow sensor.
02 sensors will keep setting off a lean code until the air problem is fixed.

i know if you don't have good air flow to the air breather, you will loose gas mileage...make sure you have a good clean air filter...this would be a good start for you...

Start your car and with the hood open, give the mass air flow sensor a hard tap. If the engine stumbles when you hit it, the air flow meter is bad. It's located right after the air filter, inline with the air intake hose.

Driving habits...incorrect tire pressure...excessive weight in car...low oil/old oil. Many things affect mileage. My 2002 max needed a new O2 sensor. Once replaced the mileage improved slightly.

go to http://edmunds.com put in the info of your vehicle then go to specs it will have all info you need

Clogged fuel filter or bad fuel pump.

First off,do not install a cheap air filter,it will restrict air flow,use a napa gold,or equal ,You can change the oil,and use a synthetic oil blend,like castrol,it reduces friction.Add a good fuel cleaner,such as lucas fuel treatment.Also have a shop set the timing on the engine,over time,the timing will change,do to wear.Clean the air intake ,or throttle body,remove all the black carbon with a throttle body spray cleaner,or carburetor cleaner.

Try cleaning the mass air flow sensor. If you don't already have a Haynes Manual for yor truck, pick one up at a auto parts store. While at the parts store pick up a can of mass air flow sensor cleaner spray. It is diffrent than other cleaners. Mass air flow sensor is usually located somewhere in the air intake.The Haynes manual will help you in locating where it is. Do not use other types of cleaners on it as it could damage it.

i think i had the similar problem with my car. I'm tell what my car did and if it sound the same then do what i did.
When you push the car hard usually beyond 3500-4500 RPM it would just sorta lock up. Almost like when you reach the Electronic speed limiter, which is 137mph on the maxima00-03. car looses almost all of the power and gains it back soon you let go of the gas paddle but same if you push it again.

i had to change Mass air flow sensor. There was a recall by nissan about this sensor. They would only fix it if your has less then 80k miles. If above this mileage limit you are out of luck. Sensor costs about 100 bucks. It is located right on the Intake pipe of the car. Which is that round black pipe. If you look at carefully you would see a sensor located right on that intake pipe after the air filter. May be around 1 foot apart from the air filter. you can replace the sensor by your self because there are only 2 screws you would have to open. very simple. My machinic had recommanded me to go get your car's computer reprogrammed after changing the sensor... but you don't have to because car's computer relizes the sensor change. i had to pay 60 bucks for dealers to tell me that you don't need to get your computer reprogrammed because it resets it self.
Down turn........ this faulty sensor told my car after 80mph that car is not getting enough air in the air system and engine just would not generate as much horse power or reach its peak. so.....because of that my car gave very good mileage on the highway. I tested it out ... It gave me 454 Miles per tank before i replaced that senor. Now by replacing the sensor my mileage went down to around 375 per tank... this number is around what nissan claims.
HOPEFULLY THIS HELPS YOU.... IT DID HELP ME.... IF YOU DON'T DRIVE FAST OR DO STUPID **** LIKE MYSELF.... DON'T REPLACE THE SENSOR AND KEEP GETTING THE GOOD MPG YOU ARE GETTING....