Goodyear G670's (and a few others brands) may have riverwear at the edges. This is not deemed a significant problem and the tire will die of old age before the rivering becomes an issue. Download the Radial Tire Analysis Guide to see pictures and determine if this is your problem.
Radial Tire Analysis
Otherwise, the problem is almost surely under-inflation. Either your gauge is wrong or you do not have the correct inflation specs for that tire and loading (weight).
Since the coach is an '05, I have to ask if these are the original tires or new ones? If original, they are 10 or more years old and have reached end of life by any reasonable measure, so need to be replaced regardless of tread wear.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL