Build your own Water Heater / Furnace Unit?

Thanks for the thoughts. I am on the West coast so temperatures are more moderate. The BTU output on the heater I don't think is as great of a concern even if it's 1/3rd what a normal furnace puts out and in fact, I think it would be more of a benefit. The issue I have is that it is a constant cycle of super hot air that cooks you out of the camper followed by a gradual cooling off a period where sometime it is comfortable and then to the point where it's cold again and the furnace kicks on again. If the combined unit has to run three times longer because it's putting out 1/3rd the heat but it's more of a constant temperature and more quite that seems like a positive benefit.

Duty cycle is a concern as well as a pressure but this is a theoretical exercise. Duty cycle I don't think I would be super concerned about, at least in my case I just don't use the water heater often enough to think I'm going to dramatically shorten the lifespan. Pressure vessel aspect is something to consider but we are not talking super high pressures. A standard ATV radiator and appropriate piping would be sufficient I should think. Unless my furnace is overly large compared to others, I can easily see a much smaller box to house the radiator/fan and connect to the ducting.