It's an automatic switch. I don't know if you have the same one as me, but mine works like this: (and yours is probably similar)
When the dot in the middle is green, the rv is feeding off the propane on the green dot side. When that tank runs out, it automatically switches to the tank with the orange dot and the dot in the middle changes to orange to let you know it has switched. You now know you have only one tank and need to get the other one filled.
Manually move the handle to the 2nd tank, and it becomes the 'green' one and will draw off that one, and switch back to the first when it runs out.
2013 Rockwood Roo 23SS
2003 Ford F150 4.6L V8