You need to learn how a hydronic system works. Best source for a vehcile based system is over at Aqua-Hot
. Read their website to see the various options. Then look in their Library
for a similar sized product to what you would have and download the installation, service and parts manuals. The part manual will tell you everything you would need to build one plus how it all goes together (or at least the old one I have does). Once you have that and understand how the set up works, then you can build a hydronic sytem for yourself. You must use fluid pumps that are made for hot fluids. Not regular water pumps. We bought ours off of Amazon. You must keep your potable water and your heating fluid seperate. Anti freeze is deadly and the "pet friendly" stuff isn't healthy. Oasis
also has their manuals online too. Toe kick heat exhangers can also be bought. Cabin Craft is the brand you see for boats. At one time, I had planned to use a couple of Cabin craft heat exchanger kits for boats located thru either West Marine or Defender Marine. That was for the Eagle 05. I had figured out how to build a system similar to an Aqua-Hot for 1/3 of the price. For the skoolie, that same exact system would have been more than the shell cost. We refuse to put a $2K (more now with inflation) heat system in a $1400 shell. Not when we can do it for less and have more redundency. Our hydronic heat system won't be finished and operational until we get someplace that we can openly work on the bus. Until then we will stick with what we have. Don't forget that you don't want to heat your bus in the summer time. Based on what I learned, we decided that, for us, seperating the systems was cheaper and easier to run & repair.
At it's simplest, this is a simple, bare bones, frugal, hydronic heating system for an RV...
The "car heater core" and fan in the image would be the fan forced rear heat exchanger from your bus. All you would need is a heat source and a hot fluid pump.
Put together wrong, a hydronic heat/potable water system can be deadly. Read up and truly understand what you are doing before you do it. If I remember correctly, to use a tankless water heater in a potable water side hydronic loop (not to heat the loop but to boost the water temp) you need to get a tankless water heater with what was called the "solar option". These tankless water heaters were designed to use already warmed water from solar batch heaters and boost the already warm water to bathing temps. They are different and more expensive. Seems already warm water messes with the sensors of standard tankless water heaters. From what I have read, the Eccotemp L5 that we own would have that problem. Otherwise it would be shoved inline between the water heater and the washing machine. As it is, the L5 we have is so tempermental that we have held off testing it to see how it will work with the washer. We will hold on to it and wait. Besides, the water here in NM is so high in minerals that I really don't want to run much thru the machine until I get a water softener on the bus.