Interestingly enough, the OSB flooring on my old '97 Viking Legend 2480 appears to have been treated with some sort of water repellant. Being Central Arizona mountain dwellers, we don't camp in the rain a lot either, but as you might suspect, it's not by choice as it may be in Southern California. We love the wet stuff, especially the late afternoon rains during the monsoon season, and the spectacularly cool evenings and mornings that follow, until the next afternoon's shower. I'm thinking you're right - it must be to protect the flooring from the road spray while towing in the rain.
