put gas in my pickup this morning and guage was working.i look down about 30 minutes later and it says empty and the low fuel light is on.it said almost half a tank after putting gas in it.i know there is gas still in tank just sure how much.anyone have this problem?an idea how to fix it?

 

Had the codes pulled on the check engine light.Shows the fuel sensor unit to be bad.Looks like pull the bed off,drop the tank or cut a trap door in the bed to fix it.Dealer item only and the price the parts store showed me was $509,is this right? Can i get it cheaper?115000 miles and its falling apart