Originally Posted by mje000
Can anyone tell me what his switch does?
If it's "on" the water heater as you indicated, it's probably the electric heating element switch and not the DSI. The DSI/propane switch is usually located inside the RV.
If you have the Suburban brand water heater, this detailed FAQ may help, especially between the DSI/propane and electric heating element options (if equipped):
Suburban's electric switch and much more
The Atwood water heaters work the same, except the electric heating element switch (if equipped) will be located inside the RV along with the separate propane/DSI switch... and not on the face of the water heater like the Suburban can be.
If you don't have one of the two aforementioned water heaters, let us know what you do have, and we might can offer better guidance.