You have a Cavagna 2 Stage Auto Changeover Regulator.
Look very closely at the 'knob'.....
You will notice a small directional arrow that is molded into the plastic knob. When the arrow is pointing to a tank and the indicator is green like the above photo, the tank has LP in it and is supplying LP to your trailer.
If the tank that the arrow was pointing to was empty, the indicator would show red.
If the other tank had LP in it, the regulator would change over automatically and use the LP in that tank, even with the arrow pointing to the empty tank.
If your regulator shows red on the side with the arrow, it means that tank is emptied and the tank needs to be filled.
