At the last FMCA convention, I had a couple of long talks with Roger Marble. He is a retired tire engineer. He was very clear that heat either from the sun or the road (gets worse with under inflation) is the prime killer of tires. The age is also a concern, but in 6~8 years there is little you can do to avoid that heat.
There is another issue that I have very first hand experience getting pummeled by. The life of most elastomerics (rubber like stuff) is not unlimited. I have had laminated pieces go bad after 8 years in the box.
Tire companies are now saying that the best you should hope for is 8 years. Michelin says that if you want to run RV tires more than 6, you should have them inspected every year after that.
I'm sticking to a 6 year plan, but I am also trading off in pairs so I don't take the whole hit at once.
Matt