Yep, I call BS as well.
Once a trailer is hooked up the average is probably about the same between all makes. Empty, I can see the Dodge(Cummins) having an advantage as it has 2 less cylinders to feed. The Chevy(Isuzu Duramax) I'm not so sure why the mileage empty is so good. Also, the Dodge(now known as Ram) and Chevy are quite a bit lighter than the Fords are.