When I first came over and had no bathroom in my chicken shed, I used a black plastic expansion tank with a hose out of it. Worked brilliantly so long as I had my shower before the sun went down.  Sometimes it was too hot.
I am thinking of putting a hose back up on the roof and preheating the water for the gas heater.
If it goes cold at night it doesn't make any difference as the current feed is cold anyway. If it freezes then I'll have ice for my G&T :)