Both 1der and I have electric water heaters......
...it takes a lot of electricity to heat water....
As it turns out, I'm pretty sure we have that exact Bosch water heater in our outside dog washing station..works great.
I bought a 2.5 gallon Rheem water heater from Home Depot for my van and removed the 1400W/110V heating element and replaced it with a 200W 12V element that's readily available on Ebay and a few other places.
There is a thread about this here (my threadjacking starts at post #16
It takes almost 2 hours to heat 2.5 gallons of water with a 200-300W 12V element.
My use case is turn on when leaving on a motorcycle trail ride and return 3-4 hours later to shower after the ride....then turn off water heater.
The advantages of using 12V instead of 110V is you don't need to run your inverter with it's associated losses. One disadvantage is that it takes much longer and uses lots of current at 12V to heat the water....and you need to run beefy wire. I had a 300W 12V element in the heater at first but the current draw was melting fuses so I backed down to 200W.
Showering once a day in the use case described above results in my batteries approaching 50% state of charge usually by day 3 of camping. I have 320W of solar and 350AH of AGM house batteries.
1der bought a small 110V marine water heater and had the element modified.....they added a 12V element to the existing 110V element. The element in 1ders setup is not a standard water heater (1" straight thread) element. The small Rheem unit I have uses a std element..so cheap and easy to play with.