I installed a very similar setup into our Defender-based expedition vehicle, and it was a terrific benefit to have hot water on demand. If I were doing it again (and I had on-board air), I would use air-pressure to run both the hot and the cold water systems.
I also fitted an Eberspacher 5kW diesel-powered engine pre-heater, so we could run that instead of the engine if we wanted hot water in the mornings!
As you can see from the second pic, I had a hot and a cold tap, and a cold-water pick-up all mounted to the side of the truck. The output hoses could be attached to a shower head or just used to fill basins etc., and the input could either be a water container of any sort, or a garden hose (on low pressure) if available. As shown in the pic, we would sometimes have the input hose coming from a container, and the hot water hose going back into the same container to heat the whole lot in a minute or so.
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