If you can figure out a way to recycle the water from a 15-minute shower, then you could be saving as much as 30 gallons of water. [Visit USGS water calculation tool here.]
An outdoor shower can be as easy to plumb as running a garden hose through a window from your laundry sink. Here is one more option which might prove effective for some circumstances.Vodpod videos no longer available.
From ecotankless.com: What makes this tankless water heater perfect is that it requires no electricity to operate. It ignites with two D cell batteries, so all you need is a water source and a BBQ size propane tank.
You can use gravity feed for this heater as long as you have 5 psi of water pressure and a minimum of 1/2 gallon of flow per minute. That’s pretty easy to accomplish, so many campers use this tankless water heater with bladder bags or 5-gallon buckets suspended from a tree.