I have a 2012 Flying Cloud that had a leak from the fresh water tank that sounds similar. Took it to dealership for repair. The low point drains are plumbed next to the pump pickup line, causing stress on the fitting, which caused the fitting to crack and leak. The dealership had to remove the fresh water tank to weld repair the cracked fitting. They replumbed the low point drains so they do not contact other lines and that should alleviate the stress problem.
If your fresh water tank is draining without your using it, that my be the problem. Or, as noted, it may be the drain from your air conditioner, which comes out a tube right behind the wheel.