Fuel Gauge
 

My 2003 GMC 3500 dually dump truck 6.0 engine and Auto Trans had a 1/4 tank of fuel last Sunday when I parked it. When I started it yesterday (Friday) the fuel indicator says empty and the low fuel lamp is lite. I went and filled up and guess what it still says empty after putting in $140 worth of fuel. I checked and both tanks are connected. Unplugging them does nothing for the gauge it still reads empty. When you first turn on the key after it sets a bit the gauge will get about 3/8 tanks and heads back to empty. if you shut it off and try without waiting a bit the needle doesn't move, you have to wait a while. I was under the assumption that if the wire was unhooked the fuel gauge goes to empty and if you ground it it goes to full or is it the other way around?
Any one have any ideas?
thanks