How about one of these?. They are cheap and work great. I think they have 2 models with different flow rates. I have the high output one and it sucks a 7 gallon water jug dry in minutes, so I'd get the low flow for a shower. No need to spend big bucks for something like this. Myself, I would use gravity.BTW, this post seems to be in the wrong section.
http://www.harborfreight.com/12-volt-marine-utility-water-pump-94639.html
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