I use these barrels as toilet flushing water since I don't have runningwater at my hunting retreat.
We carry our dinking water, and I use clean barels inside that are filled from a neighbor's well for washing up and bathing inside. I put them up on a rack and use gravity flow to a mop sink.
The barrels in this picture are joined by a 1 1/2" plastic pipe at the top. When the first barrel (Where the downspout is) fills up, it spills into the scond barrel until it is full...then it runs out on the ground. I'm thinking of adding a third barrel.
These are situated at the end of the porch, and you just take a 5 gallon bucket, dip it full, carry it inside, and dump it in the toilet tank. Then, you can flush.