We don't really have to stop with the Grail. What I always found curious about Indy's kind of supernatural is that it's not just specific type of supernatural, but the principle of "all myths are true" applies as well. Christian artefacts, Sankara stones and their roots in hinduism, Chinese mythology... all seems a fair game, even when their mythological roots (none of which are found to be a complete hoax) are in clear contradiction against each other.
So KotCS became bit of a game changer. Now we can reliably apply Clarke's third law to explain most, if not all of them. God's radio, Sankara stones, Atlantis*, and what else... Just like the crystal skulls, all simply advanced alien (interdimensional?) technology that goes beyond human comprehension - especially from the standpoint of 1930s and earlier. Those actual myths are simply a primitive mind's attempt to explain them.
*Although, this seems to be the suggestion with Atlantis in the first place. So the hints have actually been out there a while.