Any RTV work well against gasoline? Got a plastic fuel tank with a rubber gromet that a metal fuel line passes through, and it's leaking a little. I tried a new gromet, but no dice, still leaks a little. Any suggestions?

nope

Try some Barge all purpose cement. Even recommended
for sealing gasoline tanks. Doesn't harden or crack
and gets stronger with age. I dont know exactly
where you buy it but I have 3 tubes. PM me if interested.

Moroso epoxy?

ultra copper should

I have had good luck with the Mopar gray RTV. I used it on the fuel line fittings at the carb when I had no pipe dope and it sealed it up good. I have not touched it in 2 years and it has not leaked at all. Ron

Get a tube of Seal All at the parts store. That will do it.

If you have a plastic tank, it's PE
(polyethylene. There are a very few adhesives
actually formulated to bond it, and over time
the bond will degrade. I work with this stuff
every day, and an adhesive engineer. You will
find some things that will work short term, but
over time, all will fail. It's a makeup of the
resin. Many PE tanks have a spun welded fitting
on them. You might try using a clamp type fitting
and running the steel tube on both sides if you
really want a permanent seal. I did that with my
fuel cell and it worked pretty well.

Motoseal is pretty good.