There's no way you only get 14 mpg highway in a 5.0 lol, I would guess 18 or so. Heck, even my K3500 CCLB 5.7 obtained 16.8 mpg once, given it was onflat roads at 5600 ft elevation burning Supreme fuel, but still.
My 99 Tahoe would get 21-22 running about 70-75 mph on the highway. Lunati 268 flat tappet cammed (Old Style Rhoads Lifters) 305 Vortec, 4L60E and 3.42 gears. Truck was on P305/50R20s. Long tube headers, dual 2.5" exhaust to the muffler with high flow cats, tuned 0411 PCM (Lean Cruise Enabled) and 2010 Tahoe E-fans.
I had the ~300K mile 305 down to the bare block before it went into the Tahoe. New bearings, new rings, new gaskets, a new cam I had laying around and a new timing set. Polished the crank, cleaned the stock pistons, honed the block and ran. Ended up with a bad mellings lifter that kept collapsing and swapped the whole set to the Rhoads lifters as that cam was fairly aggressive in an otherwise stock 305. Added some low end torque and vacuum but kept the mid-top end power. S10 converter went from brake stalling 2,200-2,300 up to 2,400-2,500 after the lifter change.
On long hills with a 4,500 lbs trailer it was foot on the floor in 2nd gear at 70 mph going up the hill. Turning about 4,500 rpm.