The valves shut water flow from the engine to the different heater cores.
Unlike in a car, when you turn on and off the heaters or defrosters all you are doing is turning on or off a fan/blower. You are not turning off the hot water that flows through the system. In hot weather the heat the radiates out of the heater and defroster cores can add measurably to the discomfort level that comes from driving around in a mobile greenhouse.
The valves just stop the circulation of coolant from the engine into the heater/defroster cores. The radiator was sized from the factory to be adequate for cooling the engine on a hot day while going up a grade with a full load.
Most transmission coolers I have seen look like an A/C condenser in front of the radiator. I have seen a few that have the transmission fluid going through a heat exchanger with the heat being transferred to engine coolant. It can look like a box with a coolant hose going in and another hose going out with the transmission fluid going in one end and out the other.
If your bus has a heat exchanger type of transmission cooler DO NOT ever close the coolant off to the heat exchanger. Not only can you overcook your transmission you run the real risk of boiling the coolant in the heat exchanger and blowing the heat exchanger up.