The life span of an RV tire is approximately 7 years so you are getting close to replacement age. They rarely run out of tread life but rather, as you have discovered, sidewall aging. No, "small" cracks are not too bad unless there are a whole lot of them but a highly noticeable crack can be the sign that it's time to replace. There is no fixed answer but if I was you I would rather be safe than sorry especially when it comes to the front tires. Look at the video posted recently on this site showing an RV blow out on the highway.