This is something I just finished today and wanted to share it.
It is plumbed like a toilet except the water is held in the 18 gallon Rubbermaid container. The water then flows into the 10 foot long 2 inch PVC pipe that has poultry nipples on it. These were purchased off eBay for like $8 for 25 with shipping. There area total of 17 on the pipe which is enough to water about 30 chickens in the full heat of summer. The vertical pipe at the end is to allow flow so the water will always equalize into the lower 2 inch pipe. As they drink the water it drains from the 18 gallon reservoir (which is about 60% filled) and when it gets about half way empty, it fills back up to the top, just like a toilet. The only thing I will have to watch is how long the water stays in the "tank". In the summer it could get pretty gnarly if it sits too long. However I have about 30 hens in this run so they are drinking 4-5 gallons a day now so it should not be an issue. However, I think a 5 gallon bucket would actually work better to keep the water fresh.