Seven years seems to be the most commonly accepted age, but I've read some blogs on other sites and see that some tire manufactures are saying longer, Michelin says 10 years and Bridgestone has tires rated for 12 years.
I believe that the locale that the RV is in has alot to do with replacement intervals. For example someone living in the southwest would change them more often; say five years. Whereas somebody from maine,or cool climate locations would need to change them every 7 years. Tire maintence will also affect durability. Proper inflation, tire covers to name a few.