I have just discovered that you can buy Campimg Showers that are run on 4 'D' Batteries. I had only heard of the 12 Volt ones prior to last Saturday, when a friend advised me that these are now available.
Jaycar have some on sale for $19.99 each at the moment. Needless to say, I bought one today.
Don't forget to remove the batteries after use - if you leave them in you will find the spring terminals rust and corrode in a few weeks and then the pump won't work. We had one which unbeknowns to us allowed water to enter the battery compartment so we only got a weekend out of it.