Fuel Gauge

[bikernut]

my 2003 4x4 3500 6.0 with dump body has dual factory fuel tanks. It just started to show that it is empty and the low fuel light is also on. I filled both tanks as I usually do and it didn't rise to full mark. What does this tell me that the gauge is junk and needs replacing? Does this have only 1 fuel sending gauge and is it in the main front tank. When first turning on the ignition the gauge does go to about 1/8 tank or so and then quickly falls back to empty. Could this also be just a grounding problem and how do you check for either a bad gauge or bad ground. I know years ago when the gauge showed empty it meant that it was lacking ground and if you grounded it the gauge went to full does this still hold true on the newer trucks or has that gone by the way side with the advent of the computer controlled GM vehicles?