Are there two hoses that come down in that area? The one with the valve is the Fresh Water tank drain. If it is closed and there is water coming out of the other hose, you are getting a siphoning effect. The hose with no shut off valve is the breather hose for the fresh water tank. If the unit is still under warranty, take it back to the Dealer and have them reroute the breather hose to where it does not cause a siphoning effect. Post a complaint to your Dealer and Forest River that Forest Rivers designers of this breathing tube for the fresh water tank should apologize for such a poorl design and recall all units that are made this way for a Factory/Dealer fix. I complained to 'my' Dealer and he fixed it, but with time of travel and fuel used, I ended up with around a 200.00 loss out of my pocket. If Forest River does not know that they have a problem with this by now and find a fix for it then they are burying their heads in the sand with this and many other problems.
2013 Forest River Salem Hemisphere SBT312QBUD
2001 CrewCab F-250 7.3 PowerStroke Diesel
SuperChip, BTS transmission, 6.0 Trans Cooler