A friend of mine built an outdoor shower a while back. You can see it in the attached picture. He put a 55-gallon drum on the top of a stand and plumbed in a shower beneath it. If you look close, you can see the showerhead in the picture. (click on the picture and it will enlarge) The barrel is filled by a small 12-volt pump that operates off his solar power system. The system is designed to be used for hot showers in the summer.
There is one flaw in the system, he lives quite far north and at a high elevation and the water often is not warm enough even in summer. He has painted the tank black and surrounded it with clear glass to protect the barrel from the wind. He now feels that it works best when he only fills the barrel ½ way, so he is thinking about trying a 30-gallon barrel to see if that heats better.
I think the system would work very well in the warmer parts of the country. If you have any suggestions I suspect he would like them.