Many people mount their units under a sink.
RO units produce water slowly, they are usually rated in the dozens of gallons per day of production rate, so it's not like you can drop a
hose into a bucket and stand there while it fills.
They often come with a "pressure tank" that is used to store small amounts of water for on-demand use out of a little faucet at the top, but if you've got room for a bucket under the sink with a float valve, you could easily produce 5-6 gallons at a time.