You could get a HUGE propane tank and have it filled on a regular basis by the propane service people. It would probably still be cheaper than electric. And the water would be hotter. And you'd have a bigger selection of heaters to choose from.

If anyone has a need for a significant amount of hot water on demand, a number of heater models allow fitting in series. You can provide water to an entire apartment complex using the right on-demand heaters.