It's essentially an adaptation of the home units - so popular in Europe - where electricity and gas is so expensive - over there they cannot understand why anyone would want to keep 40 or more gallons of water - always hot - just to use it once in a while each day.
They are gaining market share in Canada too. On Demand water heater they call them. Not cheap.
In order to heat water as it flows tru the serpentine pipe - they must generate lots - and lots of BTUs - propane consumption will be high and I would also worry about the heat they generate for an RV