It took me a while to find my drain valve.
It is located right next to the filler hose inlet to the water tank. The hose that goes to the water pump is also in the same area. The water pump hose tees off and goes to a drain valve, located in teh subfloor where the tank mounts to. The subfloor is accessible through the galley cabinet, and i have to reach down inside a cavity to get to it. I kow thatmy trailer is older, and a different model, but maybe this helps.
Maybe if you follow the filler hose, or the water pup feed line, you might find it.