The system, now used in yachts and RVs, was originally built for canal boats in Europe.
In summary a 'heat exchanger hot water system' is a hot water tank (ours stores 22 litres) connected to the vehicles cooling system which takes coolant via a hose to the tank which heats the water in the tank. The water will reach whatever temperature the engine reaches, so after 15 minutes of regular driving you might expect 80°C water.
Of course, one rarely wants 80°C water (I'll boil filtered water for my tea, thanks), so it gets mixed with cold water for most uses and so 22 litres of extremely hot water is more than enough. When the 2 of us shower we use about 8 litres of mixed water. (We have a very nice water saving shower head).
Having this additional coolant in circulation also assists the vehicles cooling abilities, which might be handy if we struggle with hot days or steep hills.
Unfortunately, our Australian heat exchanger tank isn't as well insulated as our European one, so although it's very effective in heating water, it doesn't stay hot. With the unit in Blac we could still have very hot showers the morning after stopping overnight, but with this unit in Blu the water is just comfortably warm by morning.
And if we don't drive? Well our heat exchanger systems (both in Blu & Blac) each have an electric element inside the tank as well. So, being that we have an abundance of solar power and battery power storage we can plug the system in. By doing this we will get water at about 40°C (it can be set between 40° & 80°C, we don't want to drain our battery unnessarily).
Despite costing about a third we have found the heat exchanger systems to be much more efficient and reliable than the diesel system we had originally in Blac.