Reccomendations for 2003 with a 4.3

I have a 2003 Silverado Extended Cab with the 4.3 V6. I drive about 90 miles round trip to work, with about 85 of those miles being on the interstate at 55-65 MPH. I am currently getting 21.5 MPG, which I guess is pretty good for a full size pickup. Will a dual exhaust, K&N FIPK or a tonneau cover help me get noticeably better mileage? Which should give the most improvement?

Thanks,

Dan

I'm only getting 17mpg out of my 03 shortbed 4.3
I have a Hypertech, flowmaster 40 series single in dual out, and an AEM Brute force intake kit. I did notice more power but absolutely no more mpg's.