I just replaced all 6 of our tires are right about the 5 year mark. I found checking around the sidewall on the passenger front (exactly where a zipper blowout would occur) and decided that it just wasn't worth the risk.
I did some calculations. Figuring a constant price increase per year that is consistent between this newest set of tires and the last time that I purchased tires, buying tires every 5 years versus every 7 years over a 21 year period comes out to $160 more per year for me. I view that cost as a small price today.
Here is the way that I look at it. We live in Texas. It is a known fact here that our batteries rarely get to 5 years old. The extremes of our temperatures make Texas the highest battery failure rate in the US. My belief is that our tires are similarly affected. While someone, some place else might easy get 7 years out of tires, we may not be able to achieve the same results.
I have kept our tires covered when not in use, never use anything but soap on them to clean them and am religious about maintaining proper tire pressure at all times. In addition, we typically drive the RV a lot more than many. While all of those should help extend our tire life, I'm satisfied that our we are living on borrowed time after 5 years.
If nothing else, the extra $160 a year gives me a little more piece of mind.
2000 Georgie Boy Landau 36' DP
2005 Saturn Vue toad