Heat exchanger type water heater?

Very good thoughts thanks for your opinions.

They’re great for one-piece rigs. My boat has one.
Terrible idea for a removeable camper. Too much risk of frying your truck engine when something goes wrong, and a pain to connect every time you load.
That’s why you don’t see them in truck campers.

The main advantage that I see is it is effectively another radiator which would supply extra cooling on long hill climbs especially when you are at max load or have an older engine with some partially clogged tubes.

We have one and I like it.

I had such heater in 1 of motorhomes and did not like it.
-the heat exchanger was just pipe welded to back of the tank, so took long drive to get hot water.
-after long drive water in the heater could be close to 200F, what brought high risk of scalding
-the system took some space, that I'd prefer to use otherwise
-having make-shift engine cooling system extender makes for high risk of failure and dropping coolant on 130F desert is not my idea of fun.
7 gallons of propane last us for over a month of water heating, fridge running and cooking, so refiling propane once a month is simply enough for me.

Friends class C had these. It was nice that the water was always hot after a drive and would stay hot for a while. For a detachable TC i would not do this since the attachment is a weak link and a danger to the engine coolant. It is a risk on the class C having that much more coolant line "out there" that could leak or get broken.

Do you tend to take showers after the drive? When I drive across the country then I will stop and shower for the night, but when I am camping I am parked and this would be of no use…

Also, keep in mind with cooling systems filled with antifreeze that you do not want to have dissimilar metals. Dissimilar metals cause corrosion (chemically react to each other in the fluid). Never mix an aluminum radiator with a copper heater core for instance. Then you will have the solder as another metal also. With a water heater you might have copper, aluminum, solder and steel all in the same electric antifreeze environment and this is a problem. I think all hot water heaters are steel, but your engine block might be also so this may not matter, but what other metals might be part of the heater?

Just a thought…

Some RV's come with the "motor aide" type water heater. I think it is a great idea. If the water heater dies I'll consider adding the feature.