He was calling this a "crack" and was looking for input from readers of that RV forum.
A number of readers chimed in with opinions ranging from "Time for NEW SHOES" and " That one is ready to throw some chunks." to a discussion on what was an acceptable spray tire protectant.
The good news was that many recognised this damage as "curbing" that can occur when trying to go over a curb at a low angle.
I have outlined the area I looked at which extends well past to obvious deeper damage. Especially off to the right side where the tire just shows signs of surface scrubbing.
If I could physically inspect this tire I would take a close look at the right end (arrow) and confirm the suspect surface abrasion which would confirm no problem internal to the rubber.
Not all tire damage is from a tire "defect".
The owner did the right thing and had the tire inspected by a Michelin truck tire dealer who confirmed no defect but just curbing.