stock tire size and tire compound and pattern makes the most difference of any other component on the vehicle. tuners are only good if they are meant to increase mileage, most are meant to increase power only. exhaust helps some but not enough to pay for it most times. e3 plugs ive been putting on most all my vehicles lately, they only get slightly better mileage but the way they work they last 2 or 3 times as long as the platinum i was buying. and i have been buying them at advance you can get 20% off so for 4.80 they arent that much more than a good platinum plug and you do them half as often.
you just have a thirsy pig, ive had 2 of those engines in 2500hd work trucks and loaded, unloaded, hwy or average, i always got 11-13mpg. not much you can do unless you spend more money than makes sense to change it. its designed to tow stuff and it does.
1998 Bonneville SSE
2007 Monte Carlo LT-KYB, Clarion, Polk audio, Kicker
1999 Yukon SLE-KYB, Air lift, Reese dual-cam, Tekonsha
1987 Firebird-355/th400-nothing stock