Many RV's with dual propane cylinders come equipped with an automatic changeover valve/regulator. What this changeover valve does, is it will automatically draw propane from the second cylinder once the first cylinder runs out of propane. This is very useful, especially in the middle of a cold night, and keeps you from having to go outside and switch cylinders.
There are several different styles and brands of these automatic changeover valves, but they all operate in a similar fashion. Once you have a basic understanding of their operation/function, it should make your RV'ing experience easier. Hopefully this thread will explain how they work and will cover a majority of these changeover valves.
1. Although the styles may vary, these automatic changeover valves will usually consist of a lever (or dial) that you use to control which cylinder is the primary one, and thus the other one will be the reserve cylinder for our tutorial purposes. They will also have some form of indicator/site glass, that indicates if the primary cylinder has propane gas in it (green) or is empty (red). We will assume both cylinders are full of propane before going to the next step.
2. To operate the automatic changeover valve, you must first open BOTH propane cylinders using their handwheels on top of the cylinders. If BOTH cylinders are not open, then the automatic changeover cannot occur if the reserve cylinder is closed.
3. You will point the lever (or dial) on the automatic changeover valve to which cylinder you want to be the primary cylinder (which is the one the propane is first drawn from). The site glass will turn green, which indicates there is propane in the primary cylinder.
4. Now when the primary cylinder runs out of propane, the automatic changeover valve will internally switch over to the reserve cylinder thus drawing propane from it, and the indicator/site glass will turn red in color, alerting you that the primary cylinder is now empty.
When you see the primary cylinder is empty (the selector lever will be pointed to the primary cylinder and the site glass will show RED), you will manually turn the selector lever over so it points to the reserve cylinder. The site glass will now turn back green, since you actually now just made the reserve cylinder into the primary cylinder, when you turned the selector lever. The green indicator shows that there is propane in the reserve cylinder you just changed into the primary cylinder.
Since you also changed the selector lever to a tank with propane in it, then internally the changeover valve has a check valve that closes. This internal check valve now allows you to disconnect the empty cylinder from the hoses after you close the cylinder valve on top of it, and go get that cylinder refilled. No propane will escape due to the check valve, and you can continue to use propane from the new primary cylinder while the other cylinder is removed and being refilled.
5. Once you have the empty cylinder refilled, you will reconnect it to the pigtail/hose and open the cylinder valve on top again. This refilled cylinder is now the reserve cylinder, and once the primary cylinder runs out of propane, then the automatic changeover will start drawing from the reserve cylinder.
6. When you see the site glass turn red again, you just repeat steps 4 and 5. You can just keep alternating the two cylinders between primary and reserve, using the selection lever (or knob)
Just remember that the indicator/site glass will only show green (propane in cylinder) or red (empty cylinder) for the cylinder that the selection lever is pointed at, which we call the primary cylinder. If the automatic changeover has been drawing from the reserve cylinder, you won't have any way of knowing for how long or how much propane has been used unless you make glancing at the site glass/indicator part of your camping ritual to know that the primary cylinder is empty.
What is not fun, is to run out of propane in both cylinders on a cold, or rainy night at 3 AM.
See this FAQ for more information on refilling/exchanging propane cylinders: