Exteme fuel loss

I have a 1993 3.1 liter v-6 chevy corsica with 280,000 miles. While driving recently, I looked down to see the fuel guage was moving back & forth( as much as a quarter tank). I went to fill up the tank, thinking there was more gas than the guage said and it was exact. I plan on doing a major tune-up soon but am wondering- is this the fuel filter fuel pump or just the float?