It's not about mpgs, it's about getting the advertised mpgs, as you said. The mpgs on the V8 Rebels are generally lower than the other 1500s despite the same advertised range, and for obvious reasons (LT tires, lift). Plenty of threads about this, but not much anyone can do.
If you had the V8, I'd say get over it and just enjoy the power, like I did. But the main reason for the V6 is mpgs, so I'd tell the dealer to figure it out (knowing they probably can't) or get you a Hemi.