Easy! You may want to refer to the plumbing diagram although that is not necessary.
Find a way to pump cold water into a plastic cistern barrel that is mounted above the heater. The water should enter the top port of the barrel and exit from a bottom port of the barrel. Be sure to drill a small "weather/insect protected" hole (1/16") in the top of the barrel to avoid any pressure buildup in the barrel as it is filling.
Run a cold water line from the bottom port of the cistern barrel to gravity feed into the bottom port of the heater. Install a "T" in this line to run a cold water line into the shower. Attach a "cold tap" on this line inside the shower.
Run a hot water line from the top port of the heater to a "hot tap" in your shower. You should install a "T" in this line just outside the heater and run a "pressure-relief" line up and above the level of water in your cold water cistern. This line can be screened at the top but should be left open to maintain atmospheric pressure inside your heater. It also controls the flow of water into your heater from the cistern. Without this line, water would overflow out of the heater until your cistern ran dry. Because water seeks a common level, water will fill this line until its level is equal to the cistern water level. At this point, water will stop flowing from the cistern to the heater.
The taps installed in #2 and #3 are used to adjust the water temperature as needed.
Attach a dishwasher hose "Y" to both the hot and cold taps inside the shower and run a single "mixed hot/cold water line" to a small AC or DC pump (approximately 3 ½ gpm) and then to a small diaphragm pressure tank (approximately 3/4 litre). This pump and pressure tank, available at any RV/Boat/Solar/Plumbing supply dealer, should be installed outside the shower area. A DC pump can be powered by a deep discharge battery, which can recharged by a solar cell or simply conventionally recharged when it is depleted.
Run this "mixed line" from the pressure tank to a tap installed just before your showerhead. Turning on this "control tap" causes a loss of line pressure activating the pump to maintain water delivery pressure at 20-40 pounds to give a shower similar to conventional systems in a permanent residence.
To have gravity feed system rather than a pressurized system, simply omit the pump and pressure tank in #5 and the "control tap" in #6. All other plumbing is the same and the pump, pressure tank, and control tap can be added at any time when you desire a pressurized system.
All cold water lines can be garden hose with screw type fittings while hot water lines can be dishwasher hose again with screw type fittings. Of course you can use copper piping with soldered fittings for all lines if you wish.
You’re done! That’s it! Simple! Something anyone can do!