I have a heat exchnager
I use a Plate Heat Exchanger that I got from Duad Diesel https://www.dudadiesel.com/heat_exchangers.php and mounted it under my battery. Then it just a mater of cutting the hose going to the heater core and connecting hoses. If you decide to go this way let me know and I can give you some extra infoWhat do you use? I was looking into this system which looks interesting, similar to what @DrDmoney posted above.
https://www.joolca.com/products/hottap-v2-essentials-kit-notify-me?product=HOTTAP-Outing-Kit