Horrible gas mileage

[Rc 415]

Hey guys, I am having all kinds of trouble with my fuel system, first off the gauge does not work at all, it always reads full, until it runs out of gas, then sometimes the gauge will drop to empty or close to it. Anyway, I have the dual 16 gallon tanks, and I filled them both up 4 days ago, drove about 60 miles, and now one tank is nearly empty, I know because the truck died on me while going up a hill. Thats 3.75 miles to the gallon! what in the world can be causing this!? Where should I start to fix my problems? My motor is a basic 350, shouldn't drink gas like this! The exhaust does not smell rich either, Spark plugs are not fouled, new air cleaner, O2 sensor, oil change, lots of small stuff. Another thing that I figure I might as well mention, the engine does almost what seems like a misfire, occasionally, but I only notice it when it is in overdrive, not second or third.