No point in covering tires, what does it accomplish? If you leave your tires uncovered, you will get new tires in 7 to 10 years. If you cover your tires, you will get new tires in 7 to 10 years. You have gained nothing.
You answered your own question with your initial post. Covers did you no good.
Think through the logic of the situation. If you don't use covers, and have dually tires on the back, the outside tires block the sun from hitting the inside tires. Do people end up changing the outside tires more often than the inside? Absolutely not! Yet the inside tires were continuously shielded from the sun. Should have lasted at least another year, don't you think?
Tire manufacturers use a UV protection in the compounding of the rubber. Enough to make the tires last from 7 to 10 years. End of story.