Plough
"I am trying to figure out a way to have rain water by gravity feed just the toilets with an automatic way the toilets would go over to metered city water when the rain water runs out."
Assuming the townhouse is two storeys, the only way to have gravity fed rainwater would be to have a tank with a float switch built into the roof (we have exactly that in our small wilderness ski lodge).
In your case it would rain and fill a ground level rainwater tank. An electric pump in the ground level tank would kick in when the top tank float switch was low and would cut off when the float switch was at your high level mark.
You'd have to have an overflow on the top tank in case the pump switch ever decided to not switch off!
This way, the top tank might supply toilet flushes by gravity for a week between pump cycles?
There could be a low water switch on the ground level tank to switch to a town water top up when needed.
