Bruce,
It sure looks like you did everything right and had the right safe guards in place too. I guess this one is simply "Bad Luck". I like your idea of replacing all the tires.
One thing I would try is to inform Michelin of the tire failure. If you get lucky, they might want what is left of the tire and give you a new one or maybe a new set of 7 tires. Sometimes these matters are handled on a case-by-case basis as if it was under a recall.
If you get stuck paying the bill on your own and are good with internet sales, you should be able to sell the 6 remaining "good" tires (4 years and 14,000 miles) on Craigslist, recovering the cost of purchasing one replacement tire. Depending on your area, used RV tires can sell well. The local landscapers in my area run around with tired 6-wheel pickup trucks and bald tires. Six new-ish tires at the price of one is a gold-find to them.
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With all this talk, I looked again at the rear axle stats with our 2007 2350. Here they are for the curious.
2007 E350-SD 158 DRW
2007 PC2350 with no slideout
Actual weight taken on truck scales.
6760 - rear axle with rig empty, carrying no water, but having a full tank of gas & propane
8220 - rear axle max load condition during a multi-week trip including a full tank of fresh water, gas, tow vehicle, etc.
7800 - rear axle max load rating stated in the 2007 Ford chassis specs
In my case, the rear axle is the weakest link, not the tires. Someone had mentioned to me that the suspension work I had done when the rig was new, may have increased the rear axle rating. I wouldn't know and really can't do much more about it so I don't sweat it. That 440 overage would explain the slight rear end sag I have when on trips. Adding one more leaf spring might make it right, but I'm just living with it. If I traveled without water, I'd be in good shape, but I won't do that.
Ron Dittmer