If the knob selector is facing the trailer, you may not be able to see red/green indicator.
If the knob selector is pointed to the left tank and the tank has propane in it, it shows green on the indicator. If the tank is empty, it shows red. The regulator will automatically swap tanks when the tank is empty. When you view the red/green indicator and it is red, then you can manually move knob to other tank. At this point you can refill left tank.
Above is what I know. What follows is what I think.
Close both tank valves. Let it set for 15 minutes, or open tank connection to release propane, or light stove to use all propane in lines.
At this point, the knob is pointing to left tank and indicator should be red. (Indicating tank is empty because valve is closed.)
Slowly open left tank valve only. Indicator should go from red to green. If it does not turn green then you may have a problem. Try closing and opening valve slowly again. If you open valve to fast, the regulator thinks you have a leak and turns off gas flow. If the tank has just been filled, it may be overfilled.
If it works ok, close both valves again then go to right tank and do the same thing.