Although pricey, this seems like a boondocker's dream:
https://orbital-systems.com/savings/
In a nutshell, it allows you to take an unlimited long shower (at 3.2 gallons a minute!) but only uses about 2 gallons per shower. Cost is $3,600, so not cheap, but no more expensive than other things such as Aqua-hot, generators, solar panels, lithium batteries, etc. that make boondocking pleasant.
It would be cool to see this as an option on new RV's, and there is also a retro-fit option. Would anyone here consider replacing their shower with this?