3121 Fresh Water Tank Location

[Baldicon]

Does anyone know where the fresh water tank is located on the 3121? I know it's behind the axles, but don't know how far behind. I want to install a couple of "L" brackets under the couch, anchored the rear of the couch and the floor. There is nothing holding the couch down, and am concerned that a panic stop could cause the couch to slide forward and interfere the slide out. I want to make sure I don't drill down into the top of the tank.

Thanks in advance