Second hand $130 instant gas hot water unit .. AnMeny brand [ ebay years ago ] 8ltr per minute .. 12 lpm 12v pump, fed by a stand pipe from 4 x 25 lt plastic drums for portable water supply ..
Middle of winter in a 25 knt wind, Steaming hot shower, water flow / heat, set to give 42* C water temp, we get just on a 6 minute shower from a 25 ltr drum without recycling it .. Water resupp always close to our usual camps ..
Be aware that you realy need to set up a recirculation of water through the shower instant heater till you totaly prime the lines and heat exchanger, otherwise you waste a few liters till it fully prime and gets rid of the air in the unit .
A tub to stand in and draw water from is a good idea to extend shower time and use less water / till you want to soap up and rinse off ..