IF you cannot see your black and grey valves they could be within the enclosed undercover. It's very likely that your valves are connected to your valve handles via a cable. A notoriously problematic set up. You will have to remove some fasteners to see up onto your underbelly and determine IF you have the cables and IF so did the cable come loose from the valve. I've been there and done this one
2015 FR Freedom Express 310BHDS pushing a 2014 GMC Sierra CC
Located the valve, it is above the middle axle about 6" in from the frame on the drivers side. Unfortunately it is working so it looks like something got left in the tank from construction. It has about an inch in the tank that doesn't drain.
Max & Tina
2016 Prime Time
Spartan Toy Hauler