Where is the lever to bypass fresh water tank?

I've never had such a thing on any of our RV's... Ours appears to have an inline check valve where the city water and fresh tank feed intersect so that the city connection won't try to flow backward thru the water pump. Or maybe that's at the pump outflow...

Either way... no valve to switch. If there was, it would be somewhere around the bathroom sink cabinet, which is where our fresh supply T's into the fresh tank supply line.