1995 Chevy 1500 hard on gas

I baught this truck over the summer (1995 chevy 1500, 350) and got well over 13 MPGs then I decided to do a tuneup and replaced all spark plugs etc and my mileage went from 13 to about 8 -10 miles to a gallon if im lucky :cry:. Later found out that the spark plugs were incorrect for my engine so I replaced them with the correct plugs with the correct gap and also replaced the O2 sensor but this still did not correct the horrid gas mileage. The engine idle has always been a bit rough so I tryed fuel injector cleaner with a few fillups but still no improvment. Any help would be appreciated!