Coolant runs thru the engine, heating it up and then thru the HurriHot's radiator which has fans blowing air thru it. I just jerked the front off of mine and then removed all the hoses to vac out the fans/replaces hoses. Interestingly simple heating system. Unless you add a way to heat the coolant independent of the engine, you will not be able to use it as coach heat when parked unless the engine is running. Fine for a few hours but not a few days, weeks, months...
Basically, it operates like this with the engine in place of the water heater.
The picture here is basically how we will be setting up our rear heat exchanger from the HurriHot system, except we won't be using PV panels to power the pump. Our heat exchanger will be hung under the bus and attached to a plenum that will also serve as the plenum for the AC unit. We will use an environmentally safe antifreeze for the coolant. I've researched the stuff when we were planning a hydronic radiant floor heating system for the Eagle. Safe for the environment, safe around pets and safe for the plumbing connections and tank liners.