I suspect they are hard to find for reason: any kind of resistive device like a water heater element is way inefficient and draws lots of juice.
I have an Isotherm water heater. Like most marine
water heaters it works off of 120V and/or heat exchange from the engine
. These units are very well insulated and the mixing valve helps further reduce use of hot water -- I can warm mine up with a normal (for me) 1 hour engine
run and it is still warm (not hot) the next day (in the tropics off course).
I can run mine via the Inverter (2500W), but "HOLY AMP HOURS BATMAN!" -- it sucks the juice.