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...
My 2002 dually came stock with 235/85/16 which care 3042 lb versus the 215/85/16 which is a little over 2600 lb. I am always told when looking for tires I have the wrong size on my truck but my door sticker says different. 235'may give you more capacity and will still fit. Just a thought.