its not the sender, its not the gauge, its the mapping of the fuel sender voltages. usually you can get a dealer to reflash and fix it. or find someone with a tech2 and get it done. just have to re map and make it read somewhat closer to correct. if the stepper motor was bad it would just not work at all. if sender was bad it would flip up and down at random.

I recalibrated mine a few times. It works for a while.

My '08 does the exact same thing. It's empty on the gauge and when I fill it up it only takes 16-17 gallons.

Extremely common problem with those trucks. The only way to truly fix it is pull the tank down and drain it 100% empty. Fill it with 1 gallon at a time and hand populate the table in EFI Live with the readings.