I would think that the drain under the drawer is a solid pipe that extends about a couple of inches past the inside wall of the drawer to align with a drain hole in the floor went the drawer is in the open position.
The hole in the floor could be a simple sliding gate that opens and closes as the drawer opens and closes. This opening would either be attached to a drainable holding tank ore a hose that can be attached from the outside to divert water away from the trailer????
Good Roads
Brian & Sandi