the chemicals city water is treated with (chlorine and the like) will bubble off after a few days. That is why the those with fish tanks let water sit in a container before adding it to the tank with fish. More importantly it is why during an wide spread emergency when power is out for a prolonged period of time water authorities issue boil water alerts. So you are taking a chance letting the water sit in the FW tank. (The RV tanks are open to air. The opening is small but air is still moving in and out. Any time air come in it brings what ever bacteria it carries with it.)
For earthquakes cases of water are a better plan. For storms filling the FW tank before the storm is a good plan. For zombie attacks well.....
