It's a 2015 and probably something as simple as a loose connection of the plumbing...
You are under warranty..any chance of getting it to a Winne dealer?..or call Winne..there is a panel on one of the back walls to get to the faucet...if you pull that off then start the shower as you did and cut it off..the back pressure might be at the faucet connection as pressure builds up...you can get a flashlight to look..in my coach, the wall access plate is in the bedroom behind the bath to get to the back of the faucet..two screws hold it on...