If you have 3 different clusters, and ALL are showing that the gas gauge doesn't work.......then I would think that you have a different issue. It's not likely that all of those clusters would have bad gas gauges.
There is a common issue with the gas sending units for the carbed trucks.......the unit is grounded via the rivet that secures the grounding "blade" connector on the sending unit. Sometimes the ground makes contact, other times it doesn't.
When I have a dead gas gauge in these Mazda trucks, I can usually trace it to the grounding connector rivet, so I add another wire to the grounding wire in the HARNESS side of the sending unit connector at the gas tank.......I then crimp a standard electrical connector with a hole for screw attachment, to the new wire and run it over to the frame and secure the new grounding wire to the frame for a better ground.
The gas tank sending unit could be bad also.......but usually you can fix it yourself without buying a new sending unit. But that is a different issue, not a grounding issue. When the sending unit is known to be grounded good, and the gas gauge still is not working, or only working with a full tank, or a 1/4 tank of gas.......then the gauge "finger tab" is not making contact along the whole range of the "resistor coil wire" that it is designed to follow along. In this case, you will need to pull the sender and remove the sheet metal cover (held in place with small bent over metal tabs) and physically "bend" the small finger so that it contacts the resistor coil wire throughout the whole range of finger movement.
I can add some pics of these if needed.
