Hmm, so I guess that size would be more for a sink rather than a shower. I've been looking at the various endless hot water devices that run on propane to supply an outside shower since I have no room for one inside. But I can see a great advantage for that also, especially if you are plugged into shore power or running on generator.
Back in the '80s when I was big into off-road 4x4 truck camping, I took a marine heat exchanger and mounted it under the hood inline with my vehicle heater hose, with the radiator water flowing through the larger tubes. Then, I plumbed the smaller tubes to the rear of the vehicle and connected one side to a 12VDC SureFlow 4.2gpm pump with the pressure demand electric valve. This became the intake side of the system. The other side became the discharge side and was connected to a handheld shower sprayer with a on-off valve. With this setup, the pump only pumps cold water and not hot which will damage the pump itself.
When you wanted to take a hot shower (and I do mean HOT), you just dropped an intake hose into a bucket of water, stream, river, etc. and cranked up the truck. In a few minutes when the engine warmed up, the water flowing through the exchanger from the pump would be plenty hot to shower with even in the winter time. With the older vehicles, we found you could change the water temperature somewhat by turning the vehicle heater on, and using the temperature slide control, which would determine how much radiator water flowed through the heat exchanger.
Eventually, we found the best way to get and set the water temperature was to use a 6 gal plastic water can full of water, and keep recirculating the water through the system and back into the can until you reached the temperature you wanted. This usually only took about five minutes. Then shut the engine off and pump out of the can to take your shower. I built a nice looking control panel and mounted it under my rear bumper, next to the receiver hitch. The panel had a power switch and pilot light for the pump, and intake and discharge 'quick connect fittings' using 3/8" pressure washer fittings.
We would be camping on a sandbar on a river and freak out the canoe and kayakers when they would float by and see us taking a leisurely shower or washing our trucks off with an endless water supply. It was nice for washing dishes also.