8 years? :E Much too old for ST tires. I'm replacing our Marathons after the end of this season at 5 years old and will go with the new Endurance tires. They look like they could go another 20+ years... :(
How long they will last will depend partly on how they were treated. Heat kills ST type tires and will accelerate aging and risk of a blowout - from towing over speed rating, towing under-inflated and/or towing overloaded. ST tires can still *look* new but be due for replacement. Going up one load range on replacement tires can help a lot because it gives you more reserve load capacity (reduces internal heat). The last thing you want on a towable RV is a blowout due to the expensive damage it causes so don't take chances. 5 years is a good max and some go even less but again, will depend on how you treat your tires.
Roger Marble has some great info. on tire age on his RVtiresafety.net blog
here.
