Ive had blow outs on Previous Rv's . The tires showed no cracking. You actually have to take the tire off and inspect both sides of each tire carefully. 6 years would be the max I would keep a tire. If you are going by the dates on the tires, that might not tell you much. Sometimes tires sit a year or more on a shelf prior to being sold.
So, if you want to keep them, have them inspected by a Michelin dealer. But, your on borrowed time after 6 years.