Welcome to the forum!!
I can't point you to the plumbing layout, but they are available thru Thor at the owner's portal once you have registered. The diagram will give you a general idea, but may not show the specific location. Without seeing your coach, my advise is to look at the bottom of the coach and look for a blue and red piece of Pex tubing extending down thru the fabric covering the bottom. These will be the drains and the valves will be inside the coach directly above them. You may have to remove a drawer or false panel to get to the valves.
I hope this helps.
Kelly and Jerry Powell with Halo (Lethal White Aussie), Nash the Rat Terrorist, and now Reid, the "Brindle we have no idea puppy"
2020 Grand Design Solitude 390RK-R