This comes right from the Michelin RV Tire Guide:
AGING, WEATHER CHECKING, AND
During the pre-trip inspection, be sure to check the
tires for signs of aging, weather checking, and/or ozone
cracking—these show up as tiny cracks in the rubber
surface on the sidewall of the tire. If the cracks are less
than 1⁄32" deep, the tire is fine to run. Between 1⁄32" and
2⁄ 32",the tire is suspect and should be examined by the
MICHELIN dealer. If the cracks are any deeper than
2⁄ 32",the tire should be replaced immediately.
Here are a few tips to help you protect the tires from
these common damage conditions:
1) Keep the tires properly inflated.
2) Keep the tires clean.
3) Avoid prolonged exposure to heat, cold, or moisture.
4) Avoid prolonged exposure to ultraviolet rays.
5) Cover the tires when the vehicle is not in use.
6) Do not park near electric generators or transformers.
7) Do not store vehicle in an area where welding is being
done or in a garage that has mercury vapor lamps.
impossible to predict when tires should be replaced based
on their calendar age alone. However, the older a tire, the
greater the chance that it will need to be replaced due to
the service-related evolution or other conditions found
upon inspection or detected during use.
Tires that have
been in use for 5 years or more should continue to be
inspected by a specialist at least annually.
Those look to be OK minor surface cracks, but you should measure the depth of the cracks to be sure and have them checked.