That's a great idea. I have some 300 gallon tanks and when very dry I've pumped and hauled water from the river to water things. Filtered and treated your rain water could be used in the house.
When we moved to this farm I found a bottle-shaped underground cistern hidden in the overgrowth behind the barn, full of water. Based on other clues, it was probably put in at least 60 years ago. I pumped it out intending to clean it out - but it looked squeaky clean on the bottom! I measured the volume to be about 10,000 gallons. We found three of these, all fed from runoff from the barn and other buildings.
JKJ