Having running hot water on a long road trip is such a luxury. I tested the Zojirushi Panorama Window Micom Water Boiler & Heater CD-LFC30. It brought 3 liters of water from 50 degrees F to 212 degrees F in 30 minutes, consuming 34 Ah from my 12V battery bank through a 1200W pure sine wave inverter. Then, since I had set the desired final temperature to 175 degrees, it used about 2 Ah per hour to maintain the water at 175 degrees (or higher, during the initial cooldown from 212 degrees) over the next 8 hours. This battery drawdown is fairly quickly replenished with a DC-DC charger.
For the small amount of space this heater takes up and the convenience of being able to push a button to dispense hot water into a mug, cookpot, or basin for washing up, I think it's a winner - no need to bring and maintain a propane tank, regulator, hoses, propane water heater, water pump, etc. No need to install a heat exchanger in the engine bay and pump hot water through the firewall and eventually through a faucet. No need to think about cracking windows to vent carbon monoxide from camp stoves and being careful with an open flame inside the vehicle. And it's much more reliable and controllable than a water container on the roof rack to absorb heat from the sun when it happens to be sunny and hot outside.
For the small amount of space this heater takes up and the convenience of being able to push a button to dispense hot water into a mug, cookpot, or basin for washing up, I think it's a winner - no need to bring and maintain a propane tank, regulator, hoses, propane water heater, water pump, etc. No need to install a heat exchanger in the engine bay and pump hot water through the firewall and eventually through a faucet. No need to think about cracking windows to vent carbon monoxide from camp stoves and being careful with an open flame inside the vehicle. And it's much more reliable and controllable than a water container on the roof rack to absorb heat from the sun when it happens to be sunny and hot outside.