Fuel tank

Unless you run it right to the point it starts to sputter from the lack of fuel you will not get twenty gallons into the tank.
That said, if you are filling up when your dash gauge or low fuel light says you are empty and you can only get ten gallons in there is either a problem with the dash gauge, the fuel tank sending unit or the tank vent. The most common one would be the tank sending unit.

The tank access for that vehicle is not hard to get to, basically you remove the rear seats and carpet, remove the access plate, relieve the pressure (pull the fuel pump fuse and start the engine and let it quit) remove the fuel lines and remove the pump and sender. Then test the sender units motion with the gauge. Look in the tank as well. If you just drove it until the light is on saying you're out of fuel and looking in the tank shows you still have half a tank then the sender is bad.