Final answer:
The main supply on/off valve for the propane system is called the service valve, which is located on the propane tank and controls the flow of propane into the system.
Step-by-step explanation:
The main supply on/off valve for the propane system is called the service valve.
This valve is typically located on the propane tank itself and controls the flow of propane into the system.
It is an important safety component and should be operated by a qualified professional.