Out on Jeep runs, I have seen folks with heat exchangers.
They started with an aluminum cylinder. They put an inlet and outlet on it. The inlet and outlet was spliced in to thier heater hoses on thier engine.
Inside the cylinder was a coil of copper tubing. The output side of the copper tubing went thru the top of the aluminum cylinder and was connected to a hose that had a shower head on it. The shower head also had a clamp on it to hook to a limb, Jeep hood, whatever.
The other end of the copper coil cam thru the aluminum cylinder, and went to a hose. This hose a had a 12v pump on it. The pump was placed in a bucket of water, a stream or other water source. The pump I saw had a reostat on it, so you could control how much/how fast you pumped water.
With the engine running and warm, the hot water from the heater hose entered the chamber. With the little 12v pump running in a water supply, the coils poassed thruogh the hot water in the chamber, did a heat exchange, and provided hot water to the shower head. If the pump was turned on fast, the fresh water was in the exchanger coil for a short period, so it didn't heat up as much. At slow speed, it had more time in the coil to exchange, so it was hotter.
The system seemed to work pretty slick. IUt was simple, and easy to build.