I am under the impression that Whale makes quality products. This 12V heater is a new offering from them. The element is 300watts. (~30amps @12V DC) You won't find an 120V unit with wattage that low.

We have an Isotemp water heater in our Sprinter and it uses engine coolant to heat and also has a 750watt 120V element. We never use the electric element because the engine heats the water in about 20min drive time. Engine heat raises the water temperature in the tank much higher so a mixing valve is needed to prevent scalding. But also stays hot much longer (12 hours) once we stop driving. Best thing is there is no need to plan in advance for hot water. It's always there. Since our heater is located quite far from the sink, we use a purge valve that flushes the line back to the tank until the hot water arrives. (sensed by a thermostat)

Photo was taken prior to plumbing which was somewhat of a challenge. But worth it and no leaks with over 40K on the road.

 

Did you ever install this? Really curious, but I like the one with the heat exchanger too, I wonder if you can replace the 120 v element that comes with it with the 12v element.

Also, anyone consider installing the Atwood 4 gal model? Seems similar to the Whale one mentioned above. https://pantherrvproducts.com/atwood-ehm4-electric-water-heater-120-vac-96411/

I like the Isotemps, but the square one that will fit in my install is $799, so thinking about going with a less expensive model.

 

12 volt heating elements, if you want to try to build your own:
(it says 15 volts but it will just use slightly more power at battery charging voltages)

http://www.backwoodssolar.com/products/diversion-loads/water-load-15v-30a-or-60a-30v-30a

 

For 12V elements shop around. I bought the same one at less than half the price above.

 

A search for DC water heater elements turned up a 12 volt 300 Watt part for $24.95 on Amazon.

I think Orton built a water heater that was installed as a base for his porta-potty. Had a fabricator make a stainless steel tank for it as I recall.

With insulation to keep the water hot I can see running the element whenever the engine is on and having hot water available for ten hours or so after stopping. With a switch for running it on House power on demand it could be brought back to hot in an hour of charging using just 25 AmpHours of House battery.

I like this idea. Just have to decide whether it is easier to design from scratch (thermostat, bypass valve, insulated tank, etc.) or purchase the 3 Gallon Whale for $470 or so.

 

Yes, I'm installing basically this unit. However, I bought the 120 volt version because I couldn't find the 12 volt one with the heat exchanger. So, I got an after market 12 volt heating element and replaced the 120 volt version with it. Check my thread specific on this issue elsewhere in this forum.