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...
^Yeah those little tires get overloaded pretty quick. Stepping up in tire size is generally always preferable for heavy loads. As long as they fit and you can pull em.