Can you just follow the drain underneath to find the valve up top? It typically will be behind a removable panel somewhere near the tank.
Another solution is to ask Thor for the fresh-water plumbing diagram (use the Customer care link: https://thormotorcoach.com/customer-care/
Make sure you give them your VIN). Although asking on here will give you faster results than Thor--its a good idea to have the diagrams anyway (while you're at it you may want to ask for all the other available diagrams: Waste system, electrical, etc.)