In the past all my off grid camping I have had a 6 gallon hot water tank and never had a problem. But then when you are off grid camping you learn how to conserve water. When you shower you get wet, turn off the water, soap up, and then rinse off, this way you conserve water both cold and hot.
I always camp alone so I don't use much water or battery power unlike when you have your woman with you then it is a different story...LOL