We had a water leak in our 320RS that neither we nor the dealer could find, and it was driving us crazy. All we could see was a puddle about 12 inches wide in the cargo bay and sometimes water on the floor inside the vanity. But it never leaked when we turned on the faucets and looked. Judging by the fact that you said you had rotted wood under the vanity, you may have a leak in the same place we did. It went on sporadically for two years. Eventually, I just took a chance and removed the water supply tubes from the vanity's faucet, wrapped the threads in Teflon plumbers tape and screwed it all back together. Hasn't leaked a drop since. Don't know why the dealer didn't think of this, but they didn't. I don't know if that'll help, but when you put your vanity back in, it might be a good idea to use that Teflon tape on the faucet connections.
Dennis and Kathy
And Aggie the Gordon setter
2015 Columbus 320RS
2016 Ram 3500 diesel
Employment was great, but retirement is better