For a little further explanation. I assume the regulator is good, since your furnace works on one tank, but not the other. From the regulator backwards, you have a rubber pug tail, going to each tank. You also have the tanks themselves. All of these must flow propane. I was asking you to switch a few items, and see if it is one of them, and not the regulator. Also mentioned don't tell the dealer, and see what they find wrong.
The rubber pig tails have check valves in them, and the tank has a valve, either of which could be bad. By swapping sides with each, you might find your problem.
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