I have a 1998 Amtran RE (Rear Engine). In my bus, the engine coolant splits into two loops. One half of the system circulates back through the engine/radiator to regulate the engine temperature.
The other half of the system loops through all the bus heaters with scalding hot coolant. Each bus heater is a box with a coolant inlet and outlet. Inside, it consists of a whole bunch of loops of copper pipe and a fan. When the fan blows across the hot pipes, it transfers heat to the passenger compartment. If you turn the fan on high, it blows a lot of heat. On low or off, it transfers a little heat. After the coolant lines have run through all the heaters, the coolant has a return line back to the engine.
The bus heaters don't get much coolant flow unless you turn on the "heater boost pump." This is a 12 volt water pump that increases the flow of coolant to the bus heaters and is controlled by a switch. On many buses in warm climates, the boost pump failed and was not replaced. As a result, you'll only get a little heat from the bus heaters and defroster.
My bus has two ball valves in the engine compartment, one on the line going to the bus heaters and the other on the return. These are typically used in the summer and when doing any maintenance to the coolant system.
Additionally, I have a lever operated valve near the driver's seat to close off the line and stop coolant circulation. These valves are often defective and will neither fully open nor fully close. They are used to temporarily shut off heat, such as in the fall when you need heat in the morning but it gets hot in the day.
Bus heaters are a simple but highly effective system. My recommendation is you keep as many of the components as possible. When you need to move something, clamp off the lines, move the parts, and put it back together again. Just be aware that air pockets in the system will cause "vapor lock" and prevent your heaters from working. To fix, you'll need to bleed air or forcefully pump coolant through the system to use it again after doing maintenance - a pain but not the end of the world.