I am not an ST/Chinese tire fan at all.
However, if the OP got 5 years of good performance out of his tires, his tires were certainly not junk. He must have got the few good ones made at that time.
I do believe 6 years on any tire on an RV with typical "spurts" of use (as is common with RV's) is way more than what should be expected from any tire. Way beyond RV tire life expectancy.
I think many people look at sidewalls for cracks. And use sidewall cracks as an age indicator. However, people may overlook the fact that ozone cracking between the treads may be very present on aged tires. And tread cracks may not go completely around the tire circumference. Usually localized in an area where the tire may have ground contact on for several months. At any rate 6 years is way overdue for replacement.
The fact that he got 5 years with no problems speaks well for his tires. But I still have no confidence in ST tires or any Chinese tire.