The older Ram trucks had issues with the condenser leaking. Since it was inside the dash, literally, it was not easy to find the leak with dye. We had ours checked twice and they could find no leak, even after they checked the condenser. But we took it to another guy, and he found the leak, it was indeed the condenser, and replaced it.
IMO there should be no leakdown in just 6 years. They should stay sealed longer than that. It certainly is possible, but I would find another mechanic to check it out before I accepted it as fact.