I found quite a few threads searching with "solar water". This one :
https://permies.com/t/57410/Solar-water-heater-plans-safe Seems to discuss quite a few ideas.
Whatever you do you'll have to think about winter cold as well as solar heat.
I'd really love to add a solar heating system onto our property. Unfortunately all the useful roofs face E/W, which makes it less viable. Although in the summer (when we run the cooker which heats the water less) there is much more angle of sun and daylength. We are 57+ degrees North.
I have used copper pipe around the flue of a wood heater.
I wrapped a full roll of 1/2 inch pipe and led it to a gravity hot watertank
I am not sure a LPG heater will have the same ability to heat the ater
John Daley Bendigo, Australia
The Enemy of progress is the hope of a perfect plan