My dream on board water setup has been planned in my mind for years, but I haven't pulled the trigger to make it happen. Basically, move the spare tire out to a tire carrier swing-arm bumper. Install a large ABS food-grade tank (13 gallons or so) up underneath where the tire was, and design a thick metal skid plate to protect it. Then I'd run a 12V pump (Shurflo or similar) to draw water from the tank, with a "Y" fitting after the pump to split into two separate runs. One would become the "hot" line after it runs through a plate style heat exchanger connected to a coolant line from the heater core. The other would remain as the "cold" line, and the two would meet back up at a thermostatic mixing valve that combines the two into a single shower-style nozzle. The thermostatic valve allows you choose how much of the hot and cold water you receive out of the nozzle at the same time, so you can control it for things like washing dishes or taking a shower or spraying off your boots.
The big hangup is that I am not the hugest fan of having a tire carrier. I had one previously and I sold it because it was not an ideal setup for me. I get into my hatch area A LOT for my side job, and often times in weird locations that don't always allow me to fully open a swing arm. There was more than one occasion that I had to climb over the rear seats to get my gear out of the cargo area.