Thanks for the assistance folks.
The best I can tell you on the weights is the rig runs around 15k (depending on how much water/wine we carry). I run 95-100 lbs of pressure in the tires, checking before every run, no matter how short or long. I do have a tendency to be a little heavy footed - running 70ish in the 75mph zones. I rarely exceed 75 unless there is some need for passing, etc. Except for backing in/out of sites and storage, I don't think we do an unusual amount of tight turning. In fact, we typically book pull-thru sites.
We bought the rig new in 2011. In a few short months, the stock tires started bubbling (Duro brand), so we did the research and replaced them in late 2011 (we bought 5 to cover the spare too). I just spoke with the shop I bought them from, they explained the tires were made in 2011.
I haven't been to storage to check the dates on the sidewall yet. Nor have I had a chance to get pictures.
The tread wear seems even on the remaining 3 tires on the ground. It just looks to be about half the depth or perhaps less than the spare (which had never been on the ground). Maybe that's normal, I just seems like a big difference to me.
I am surprised that we've worn that much tread. I will check the alignment and other items mentioned here - thank you for the thoughts.
Any input is welcome, but for now, I'll have to replace the blown tire, then check this other stuff out. Once we get to the bottom of it we may replace all the tires. We have longer trips planned this summer and want the peace of mind.
We appreciate you all.