Tire Dealer trying to sell me older tires, what do I do?

So you are willing to accept 6 or 7 months but 12 is too much? IMHO you are overthinking the problem. These tires spent those unacceptable 5 months stored inside, not exposed to sun, moisture, asphalt or road debris, and without 5-10 tons sitting on them while under-inflated and often parked in the same spot. I would put them on the coach and save the receipt so I knew when it was time to think about replacing them again.

Dave