I have the Marshall.
When a tank runs out, the indicator goes red. You can see from pic that one tank is empty and one is full. The lever indicates which tank is empty, but the auto switch over is feeding from the full tank. This is especially handy in the middle of the night when running the furnace
or when it's 45 minutes on a gravel road to get propane.
I can then close the tank that the lever is pointing to, move the lever to the full tank, remove the empty and take it to be filled while the full tank continues to run the fridge
If you are going to replace these parts and the tank, I would replace with auto change over.