Why not introduce a cistern type system for your rainwater tank with a float valve, and you wont need a switchover system when the rainwater tank gets low. When the float gets below a certain level it will open the valve and water from your main supply will fill it up to the level you set to switch off.
This way, you will always have water in the rainwater tank, and perhaps, the odd bit of chlorinated municipal water will keep things in check!
For as long as you have rainwater keeping the level in the catch tank, you will not be drawing from your house's municipal supply.
Does that make sense for you?