I have the same problem. My sender is a 0-90 ohm sender. I disconnected the sender wires and an open on the sender wires causes the gauge to read full, a short across pulls it to empty. I added a variable resistor and varied it from 0-90 and my wife said it moved the gauge 1/4, 1/2, 3/4. So it is my sender. I have to burn some fuel off before I can change it. The gauge does give a hint of the volume sporadically - I think I have 3/4 tank and I need to get it to around 1/3.
Rick and Larrie Dee
1997 40' Newmar London Aire DP CTA 8.3 (Mechanical) 325 Spartan MM
Bringing her back to her glory.
'08 Jeep GC Overland.