More and more overlanders want to have a hot water supply on board. I don't need it, but I understand if someone wants it. There are many solutions. What I would never do, however, is to change anything in the vehicle's cooling circuit for hot water production. This is where something that absolutely must work without any failure meets an equipment item of comfort or convenience. Namely, the cooling of the engine to the hot water supply for the shower or whatever. And if something goes wrong with the engine cooling, you have real problems on the road under certain conditions. So I try to stick to the rule: keep it simple and and little error-prone.
If anything, I would solve this with a separate heat exchanger that can be easily disconnected somehow. Much better I find - if it is to be at all a "complicated" technical solution - an electric hot water boiler or one that is coupled with a warm air parking heater. By the way, I keep it very simple, I take a black water bag (I had already mentioned elsewhere here). In it, the water in the sun becomes warm (or attention: even very hot!!!). That bag lies at my rig well fastened on the hood. At that place warms even the engine waste heat a little. Alternatively, I simply heat water with the kettle on the stove or fire and mix it with cold water from the water canister, so I can have pleasantly warm shower water.
This is just some food for thought. Everyone has a different comfort level and sometimes we overlanders like to try around with DIY solutions because it's fun. I understand that!
I'm curious to see how you implement it in the end. Please show us your solution here if you like!