I just got this truck. It has the V-6 A/T. What kind of MPG can I expect from it in City/ Hyw. Are there any problems I should look for? So far it feels and looks like a great truck.

For problems,

Most important is the ball joints and front rotors!!!
Check to see if the front passenger carpet is wet, which could mean the heater core is shot and leaking coolant in the cabin.

Also since you just bought it, I would change the rear end fluid, steering fluid, tranny fluid and coolant. And what other things that might be needed. ie: like the belt and plugs/wires

The carpet is dry and the ball joints looked ok. It is a 2wd. Rotors are good as are the front pads. It needs a new serpentine belt. I did not see anywhere to put a breaker bar on the tensioner. Do you put a wrench on the center bolt on it? I will drop new plugs and change the fluids as soon as I get a chance. can you tell a difference with the K&N drop in filter on these?

With the belt there's a tensioner present. All you need to do is move it with a ratchet for the belt to come off. There should be a diagram under the hood on the radiator cross member.

I don't have a drop-in K&N but I do have a open cone style filter. You will feel a slight increase in power when accelerating. You could also see a 1-3mpg increase in mileage.

The V6 isn't that bad on gas, I've seen guys get 16-22mpg on average or as high as 27mpg on the highway. It all depends on the condition of the drivetrain and the driver's right foot.

Change the fluid in the rear end? Why?

It should be serviced every 30-40K. It doesn't hurt to open it up and clean the magnet and check the gears for any damage or tooth chips. It's very cheap and easy to do!!