I think the Ford will give you the added gears in the transmission you were looking for while the GMC 6.6 gas is still stuck with the 6 speed. I have the 6.0 gasser and the six speed which for me is fine. The new trucks with the 6.6 gasser will only have a 3.73 rear end and what GM is saying is that the larger engine makes up for the gears. I preferred to have the 6.0 with the 4.10 until GM figures out what transmission they want to put in the newer trucks. The Ford may surprise you with what you want at a lower price than the GMC. Overkill may be a good thing with an RV.
Curly