When I fill the van with gas the gauge goes to the full indication but after using about 1/4 of a tank (I think I have a 30 gallon tank) the gauge begins to move up and down randomly. Once I have used about 1/2 of a tank the gauge shows empty but still "bounces" a little.

How do I determine if the problem is the gauge or the sending unit?

thanks,
mike

How do I determine if the problem is the gauge or the sending unit?

I would tap into the sending unit wires with a meter and compare the readings again the dash gauge.