Tire brands and life expectancy for 5,000 pound TT?

Nearly all RV tires get replaced because of age, rather than wearing out. The simple truth is that all rubber tires, belts, and hoses get hard and crack with age. So the short answer to your question is, yes, the 8 year old tires on your RV should be replaced, and no, there are no longer-lived options available. Failing to replace them risks -- and ultimately guarantees -- that they will fail in service with a blow-out (or if you are really lucky, they'll turn up flat one morning).