I don't remember the exact specs, but for shortish showers every few days the propane use doesn't amount to all that much. (Edit, just checked the website and 20# propane will produce ~ 1000 gallons of hot water).

We don't catch rain water or make water, but at sea we wash in salt water via a pump at the galley, and use a spray bottle of fresh to rinse the salt off the dishes. I used the space where the engine hot water heat exchanger used to live, and added a 30 gallon tank, so we now carry 130 gallons in 3 separate tanks. I've never found I needed more than that for general cruising and am not sure I want to carry too much more weight on my boat. I also don't want the complexity of a watermaker. We all have different priorities, this is what I've found works for me.

(On my Westsail 32 in the Pacific in the 80s I did catch rainwater, and it worked great in those circumstances)

All the best,(nl)Max