It's not quite that simple. Along with increased tire size, one has to ensure wheel size and offset is proper to keep the rubber from touching especially under load. Some of the 2nd generation guys have successfully run P255 rubber on stock wheels at a trade of load handling (E to D). To increase to a 265 or 285, a larger rim should be used with an increased offset again to keep the rubber from touching. Touching generates heat rapidly, and that is very bad!
I'm no expert nor know the specific numbers for the right tire and wheel combinations. There aren't a whole lot of options or choices out there for duallys. There are some 6. 5 dually after-market wheels available in 16 and 17 inch sizes. They are available around the country. Many dually owners up-size to 19. 5 medium duty wheels and tires in the F and G load ranges with more variety in sizes. Rickson has a direct bolt on for 8-lug applications or there are 10-bolt adapters bigger truck wheels. A couple of other direct bolt wheel suppliers are out there as well. Do a Google search or search the TDR. Lots of great information here and on the www.
BF