The dually 3500 should have a great cargo rating, especially if less than 3 years old. However some of the 2000 and earlier Dually GM's and Dodges only had a 10,000 GVWR, about what is found on a 2005 and later F-250 pickup with single rear wheels. The current F-350 dually has a 14,000 GVWR, and cargo rating above 5,500 pounds.
So what cargo rating do you have? Or what model year and GVWR do you have? The diesel has a 600 pound heavier curb weight than a gas engine pickup, thus can normally carry more with the gas engine.
Good luck on your search!
Have you checked
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Fred.