If you are already spending the money for the Off-Road Pkg you might as well spend the $400 to get the BT Pkg. As others have mentioned you will already be on the wider OR tires and have the lower final gear ratio, so the only diffwerence in mileage would be as it relates to the extra drag on the big tow mirrors and I suspect you would never see that in gas mileage.
Big Tow, a lot of stuff for $400 or less...
Big Tow Package (SE and LE Only)
• Class IV receiver hitch
• Additional 7-pin wiring harness plug
• Trailer brake controller pre-wiring under instrument panel
• Heavy-duty battery (710 CCA)
• Lower final gear ratio (3.36:1); similar to 4.10:1 on a 4-speed automatic
• Extendable tow mirrors
• Transmission temperature gauge
• Increased tow capacity
SE: Requires Trac Package (4x2 only). In addition, SE Rockford Fosgate Package or SE Popular Package is required.