If you try to "add" something to the factory "made" fittings, it's a pain to seal those. You can't simply tap a threaded fitting.

If you have prices for a quality MARINE tank, unless the price was a lot less, I'd put that in for a fuel tank.

With a fresh water tank if it leaks a little because you overfilled it or did something, it's a pain in the neck.

If you spill diesel, it's a lot worse.