My greenhouse has integrated gutters in the extruded aluminium channels, so it was easy to hook up a barrel system using 200 l plastic drums. There's a primary collector on blocks next to the structure, and the outlet from that goes into the greenhouse, which has an excavated floor. Inside there are three more barrels daisy-chained together to act as a single tank, The gravity feed from the outside barrel is attached to a float valve on one of the inside barrels. All the inside ones are black to maximise heat gain (but since they're back in a corner and covered with plants it's not like they're bathing in solar rays). I also manually fill a bathtub with water from this system, so all together it's about 1000 l of storage and it's quick to do a round of watering when we're dipping the can or bucket out of the bathtub.
So far, in three years of operation, we have not needed to use town water in there once. Everything we've needed has come from the sky. The other thing I've done was to put most of the plants on tables covered with corrugated roofing iron on a slight angle which drains into buckets. This lets us reuse as much water as possible. Can you tell I spent most of my life in a desert?
I'm also a fan of not allowing the humidity to get too high in greenhouses, as that's how lots of pathogens get the advantage. Capturing the runoff helps us control moisture levels.