I've had very bad luck with tires on my dually due to separation issues. I'm only getting 15k to 20k before this happens and 3.5 to 4 years. I only have the camper on the truck about 3500 miles a year...
I had separation issues on my Dually. PO had put aftermarket wheels on an it put the sidewalls of the rear tires too close to each other and they would overheat. There really should be well over an inch of space between the closest part of the two tires under load. I ended up putting in a spacer. I haven't had a blow out in 5 years since I did it vs one inner dually a year on the right side ( high crown loads).