rainwater tanks are built on to a wall high to collect rainwater

Depends where the tanks are. Tanks on the ground need a pump. Higher up on a wall ya gotta do the sums. Water is heavy, and how strong is the wall. Or is the idea to have a tank on stilts?

Then there is the tank cleaning problem. Harder higher up.

Sometimes simple ideas ain't so simple long term. But if these foibles are considered at the outset...............