I get that the idea of a recirculating shower isn't for everyone. But idea of a long, warm/hot shower with ample water flow sounds pretty apealing to me--especially on a cool/cold day. And, especially if it could be done with little fresh water use, low energy use, and little water going into holding
tanks.
I've looked into recirculating showers quite a bit for a future Vanlife build. There are lots of examples of Vanlife recirculating showers on YouTube, but I think they mostly get it wrong--They load a closed system with ~10 gallons of water and then a complex filtration system with the intention to re-use the water for days/weeks. Longer term use requires replacing/cleaning the filters, UV for help keep it sanitay, dealing with clogs, and using special soaps with no scents (filters don't remove scents.)
I believe a much better approach is to (re)circulate as little water as possible (like less than a gallon) but to change the water once or twice during a
single person's shower and always completely change it between showers. Every shower starts with clean water and ends with an empty system. Because you replace the water whenever it's dirty, the need for filtering is drastically reduced. Also, recirculation also dramatically reduces the energy needed to heat, possibly making the system capable of running from electric/battery.
A system like the Hour Shower uses very little water in the system and a
single, simple filter. I believe one could have a high-pressure, high flow, endless, hot shower for just a few gallons. Version 2 of that system system uses a
heat exchanger to (re)heat the recirculated water without any contact/interaction with the house water system, a temperature controlled mixing valve to keep it at the perfect temperature. In use, I would expect to rinse off to get the majority of dirty off the body with water going down the drain, then start the recirculation and stay in as long as you want. Maybe change water again after soaping. A small
electric water heat could be run from
battery bank and provide ample heat as long as it's being recirculated.
Again, it's not for everyone, but I believe it could be done in a way that provides a benefit without too many downsides.