There is no law or hard rules for tire expiry date. Michelin recommends changing after 10 years, many people change them after 7 to 8 years, some after 5 year$$$. Some brand of tires develop sidewall cracks way before 10 years especially if they are exposed to sun for many months and not rolling, but cracks does not necessarly means the tire is due for replacement, you have to see the recommendation for the specific brand.
Legal wear limit is 2/32 of an inch, I would not go below 4/32. A new 22.5 inch RV tire normally have between 17/32 to 22/32 depth tread, so you have a lonnng way to go before they wear out.