I'm guessing most of the problems with foul-tasting or -smelling water come from microscopic critters living in the tanks. I had some foul smelling water (never would I drink the stuff), and referred to Peggy's excellent book which covers other topics besides waste. She has a one-page recipe for treating fresh water that involves shocking it with a bleach-water mixture, then flushing with fresh water. It worked for me, and the rest of the season was fine. I've not had to shock it this season, but I know where I'll look for an answer.
I debated running the input water from the marina hose through a in-line filter like they do for RV's, but I think I want to keep the municipal-level of chlorine in the water to limit bacteria. Perhaps Peggy will answer that, if she weighs in on this post.