[fuzzypuppy]

Your getting some pretty poor mileage especialy considering there are so few miles on the engine, I regularly achieve 18 mph on my heavier larger truck with a 5.7 that has 200.000 k on it.
All the modifications you mentioned will help a little after each is installed and will compliment eachother.
But unfortunatly all in all there is no one item that will give you all the gains you seek.
The non OD trans your running is certainly a drawback in your wish for better mpg, and the engine your running has never been known for it's high performance qualities but since it's all stock you have no where but up to go.
The one good thing is with only 28k on the engine you have alot of time to play with it.