Do you find your bathroom to be cold in the winter with the corrugated steel walls? I am close to finishing my tiny house (not on wheels) and have been thinking of using the steel for my shower. (Now that I see your bathroom, I might consider doing the whole room–I love it!) My bathroom exterior walls face N and NE with R30 sprayfoam in the walls and R42 sprayfoam in/under the floors; the wall studs are off-set to eliminate as much as possible cold bridging. I live in the east coast of Canada (Maritime Provinces)–cold and damp here.
I have a heater in my bathroom, so the metal actually holds the heat really well. It’s the warmest room in the house! My dog loves to lay in it.
do you still have the link to the tub you used?