I have a 2006 Forest River 221 edition. My floor plan is different from yours (see https://sprinter-source.com/wiki/index.php/MB_Cruiser) but in the compartment holding the water tank (located right behind the driver's seat), there is a shut off valve that drains the tank. The plastic diverter valve is at the base of the tank. The water line that feeds the fixtures from the tank comes out at the bottom of the tank. Where the water line comes out of the tank there is a diverter valve that when turned, drains the tank. The diverter valve is similar to the one listed here : https://www.amazon.com/Flair-16902-Plastic-Degree-Center/dp/B00IRUN9R6?ref_=ast_bbp_dp.
If a photo would help, post again, and I'll get one.