Found this via a van conversion site. Might put one in my 22. I would need shore power to run it, but it could be run from batteries if you have enough.
https://www.amazon.com/1800W-Electric-Tankless-Heater-120VAC/dp/B00DBS8MT4
https://www.amazon.com/1800W-Electric-Tankless-Heater-120VAC/dp/B00DBS8MT4