Thanks for the input. I think I've been pretty diligent about proper air pressure. At 12,000 miles I had the brakes & wheel bearings checked and they seem OK.
Anyway, I ended up replacing the tires and the Tire Plus dealer said they recommend over inflating trailer tires. He suggested 50 psi which is 20 over the recommended pressure. Not sure what to make of that.
He also talked about lighter trailers and bumpy roads leading to greater tire wear. The only really bumpy road I recall was in South Dakota where I slowed down to 35 mph for a 390 mile stretch.
I'll be more observant of abnormal tire wear with these new ones.