We had a previous boat that had copper plumbing. I replaced it all with Sea Tek pex plumbing. Was very worthwhile, better flow due to larger tube ID. Also much easier to work on.
As to the metal sewage plumbing, what a bad idea. I would replace it with Raritan SaniFlex hose. It is extremely flexible and doesnít permeate easily. Then the holding tank should be plastic. Ronko makes a heavy wall tank that are a very good quality. They have hundreds of standard sizes and will put the fittings where you specify. Great product. No affiliation.
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If you arenít one, there is no explanation possible.