Auto-changeover
The auto change-over valve, as I understand, will change from one tank to the other when that tank becomes empty. or when the LP pressure drops. If you check the red-green indiacator and it is red, then the tank that the valve handle is pointed to is empty but you shouldn't be out of gas, you will be running off the other tank. You then flip the handle to the other tank, the indiacator should turn green. You can then remove the empty tank and have it refilled.
Now, here's what happened in my case the first time I had to deal with this. We run out of gas one cold morning. I went out and the indiacator was red. I flipped the handle to the other tank, indiacator stayed red and no gas. As it turned out both tanks were empty, my auto-changeover had allowed both to be used evenly, so both tanks were empty. I called Heartland and they sent me a new auto-changeover, and the new one did the same thing. So now I keep the spare tank valve shut off until I run out, then I flip the handle and turn the tank on.
To see if yours is leaking remove the gas line from the tank your not using. Turn the tank on that the flip handle is pointed to, then put a little spit over the small hole on the gas line that you removed from the spare tank, if you have some gas coming out the vave is bad.