Unfortunately, I've been on many boats where AF was used in the fresh water system and it never seems to fully come out. Sorry. I only add the downer to encourage you to consider replacing the lines to the galley and the head. If they are old anyway, you may appreciate the encouragement when you dig them out, as they might look a bit scaled anyway. It seems as if the prior owner left the AF in there for a long time? Just be sure to use line that was designed for fresh water.
If the tank material is metal, you can probably wash it well. If plastic, not so sure.
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In the harsh marine environment, something is always in need of repair. Margaritas fix everything.