My Friend Was Helping Me Put Gas In My 1989 32' Wellcraft And He Put Gas In My Water Holding Tank . I Have Flushed And Flushed, I Still Have A Gas Smell Even After Treating With Chlorine.
I really believe for all of the time and effort involved in trying to flush out that tank you'll be better off in the long run to replace it.
Definately sounds like a harsh statement but once that plastic is permeated with any type of fuel it's pretty much a goner.