I would place an adv in Craigslist asking for a ten year old unit that will last 3 times longer than the new units... Per your theory of old vs new.
You're being jacked by that dealer. Jacked to the point, I, and other logical types here aren't sure you had a real unfixable problem to begin with.
It's obvious someone isn't being square with you.
Wires is wires and thermostats are 38 bucks and up for a digital if indeed you need a thermostat to set compatibility.
What did they say at the place you went to for a second diagnosis/estimate?
Thor has NOTHING AT ALL to do with the ac unit other than a low paid guy stuck it in the roof hole.
Take a deep breath, apply some logic, quit listening to the guy with the tiffin.
We're here to help. The tiffin guy, not so much.
Tell us what we can do or say other than the concensus of you being jacked with, and we'll do our best.
Thor is blameless.
The best thing about posts like yours is it allows people on the fence about buying an extended warranty a bit of ammunition in favor of the purchase.
(we're not a lot of people on this forum, we're a LOT of people on this forum. We've not heard what you've heard)