The website,
http://www.fallingwater.org/1/home doesn't appear to have any special parking info that I can find. It does have a link where you can click to send a message and also a Contact Us link.
Tour groups can come by bus, so there must be parking for large vehicles somewhere. I looked at the satellite view on Google Maps and the main parking lot seems to have only cars in it. An empty lot with very long parking spaces shows further down the screen.