With that many miles, did you hand calculate your MPG? I just delivered a 33' Weekend Warrior to Salt Lake, Utah and was getting around 9. 5 MPG on the over head but that was mostly uphill all the way from So. Cal. and I did not hand calc. my MPG. Although mine is a G56/3:73. I kept it in the 23-25 RPM range no mater what gear I was in, when climbing the grades and it did pull strong. I like running high RPM to keep the EGT down when climbing grades. I felt the (305 HP version C&C) difference in the lower HP than my 04. 5 pulling heavy, but that was only when passing the slower semi's when they passed another semi and entered my lane slowing me down because traffic would not allow me to change lanes.I have an 08 with the 68rfe and 3. 73's.
I just pulled my 14. 5K 5er around 7000 miles. I clock around 1500 rpm at 60mph just fine. If I need more power I drop it in to 5th and not a problem.