I don't know how recent his numbers were, but John Oliver did his latest show on RFK and the numbers were pretty alarming. Add Jill Stein to the mix too and it's a bit worrisome.
I see it more as appealing to independent/undecided voters. Disenfranchised, disillusioned and dissatisfied people who aren't satisfied with the two party system. If you're even considering voting Trump at this stage I don't see much changing your mind. I think a lot of these voters would have either voted Kamala or not voted at all given only 2 candidates, but Stein and RFK are giving them a black hole to toss their vote into instead.
"Elsewhere, an April CNN poll showed Kennedy with a much higher favorable rating among Trump voters (42% favorable vs. 16% unfavorable) than among Biden voters (19% favorable vs. 53% unfavorable), while a Politico analysis revealed that Kennedy has raised about twice as much money from 2020 Trump donors than from 2020 Biden donors." (source from end of June).
So more Trump voters are looking at and donating to RFK than Biden (at the time) voters. I'd be interested to see new numbers with Harris as the candiate.