Poor gas mileage

V8 Four Wheel Drive Automatic

There is no check engine light on. I've changed the fuel pressure regulator. I didn't seem to be leaking but changed it anyways and it still doesn't seem any better on gas. I put injector cleaner in it every oil change 5000km's. Can the return fuel line be plugged? I've talked to the previous owner of the truck and he told me the truck just stalled on the highway and wouldn't start up. He got it towed to the GM garage and they changed the fuel filter and pump. It worked after that.