My '02 (no aux tank) would do the same, every tank. Fill it up, needle goes to empty, low fuel light on, use 1-2 gal, then it goes back to full and works normally.Sometimes when you have lots of fuel in the tank-- from extra tank draining into the main tank or Aero big tank like mine, the gauge will go to empty and the fuel light will come on. I believe that it is caused by the contacts in the sending unit grounding when it is really full. After you use some fuel, the gauge will go lower and will read the proper fuel level. Mine has only done this 4 times this year. I doubt that changing the sending unit will correct the problem.