There's some flawed, and straight up wrong information in this post.
It most definitely can be done, and easily at that.
This is a pay to play type of thing. Good things don't come cheap.
I won't argue paper specs on here, like so many seem to love to argue about, but i will argue real world stuff all day long...
2 different friends have the Isotemp Slim Square water heater which had a 115v 750watt element and a 4 gallon tank in their rigs.
These are standard equipment on the Global Expedition Vehicles, and many yachts. Both of them said, just with the heating element, and not the hydronics running through it, they have the 4 gallons heated up for a hot shower in 30-40 mins.
Math: 65amps +/- at 12v (6.52 at 115v) for 30 mins is a total of around a 32amps +/- draw (12v). Correct me if i'm wrong.
This is almost a non-issue for anyone with a decent size battery bank. Personally, i have 1 300ah lithium right now, and on the fence if i am adding a second.
I have 750 watts of solar and a 150/70 controller and a 60amp Sterling DCDC. If 100% cloud cover, and zero solar input, i could top off the battery easily idling it for a short time. But doubtful i ever would need to, but the option is there.
Its pricey ($800-ish) but its low (relative) draw and dimensions, make it ideal for a lot of applications if ya have the $ and deem it worth it for your application.
For me, the math, dimensions, and draw make sense to invest that much $ in a water heater that will take up very little space and has an excellent reputation and been around for years.
Some my say its too expensive. I've learned YEARS ago, an ounce of prevention saves a pound of headache, cry once...