Thinking back now to some of my older projects... I have used (clear) RTV to seal where the fuel lines come through the firewall, including a few ARFs that have that huge hole for the fuel tank neck that ends up not being used when a larger tank is used.

After years of use, the RTV ends up clouded and... I'm not sure how to describe it, but it is definitely some kind of break down.
You know how "fresh" RTV is very flexible and stretchy... you can maybe cut it with a fingernail by pinching, but when older, it can be broken apart with the fingertips like very strong Jell-O.

If not disturbed, it still works fine for sealing the firewall, but I don't think it's something I'd want to use to seal a wet part of the fuel system.

Don't know why it took me so long to remember that.