I'm assuming it isn't that unusual or the pump out guy wouldn't have had an adapter. Maybe they could tell you what the thread pattern was and search for one on the internet.
Another option is to swap out the deck fitting for something that takes a cap you can more easily locate. You may find a marina or two along the way that will help you drop that cap in the drink, so having a spare is helpful.
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In the harsh marine environment, something is always in need of repair. Margaritas fix everything.