For showers and laundry I use a Swedish army surplus stainless steel pot on my Wallas. It takes about 25 minutes to heat enough cold water for two decent showers.
It’s not instant but about the same time it takes to put my shower tent up, grab some clean clothes and crack a couple of beers.
If I‘m eating a meal I know the washing up will follow!! BoiIing the kettle while eating is a bit of a no brainer.
Instant isn’t a necessity by a long shot.