I have a 2017 3500 dully and I want to install an auxiliary fuel tank because I pull a 5th wheel. I have an RDS tank and install kit from Northern Tool and have a question about the shutoff valve. Do I leave it open or closed when driving? What keeps it from overflowing when the main tank is full? The instructions also say that some but not all trucks have an open vent on top of the main tank and to run a hose from this vent to the height of the auxiliary tank. Is this to keep it from overflowing or what? Any help is appreciated.