The DHC 3-1 is a 120 volt that draws 25 amps and requires a 30 amp breaker. That is right at the limit of what you could use, and you would have to pay attention to the other electrical draws to keep from tripping the main breaker. I would rather have a regular small water heater for a couple of reasons. One is that it limits your use of both resources(gas or electric, and water) to the size of your water heater tank. This limits how much grey water I produce(and store) and how often I have to visit a dump station. The fact that I can't run the hot water indefinitely is sometimes the only thing keeping me from doing it(Aquarius) The second reason is that I see the same disclaimer on many on-demand water heaters that says that the temperature will vary, that there is a minimum flow rate at which the heater will activate, that you have a greater chance of scalding yourself, and so on. It just turns me off.
I am looking at getting a shower sprayer that just puts out a heavy mist of water and has an easy to use on-off switch. I have a friend who uses a similar thing for a outdoor portable shower and uses only 1 to 1-1/2 gallons of water per shower. You boondockers are doing the math right now, I'm sure.
I also noted that my 45 year old bowen water heater got ten gallons of water up to temperature in less then 1/2 hour. I would hope that a new one would be a little better then that even.