you don't really need a manual for the heaters. They are dirt simple. They consist of three parts...
An electric fan (or fans)
A small radiator (or heater core if you prefer)
And a metal housing/shroud including duct work that directs the airflow through the heater core and into the bus.
It's very simple. Hot coolant flows through the heater core, the fan forces air through the radiator which in turn heats the inside of the bus.
Engine coolant is THE best solution for heating the interior of the bus while driving. That heat is free. If not utilized, it is vented to the atmosphere.
Quis custodiet ipsos custodes (who will watch the watchmen?)