I currently own a V10 SMB and absolutely love it. I have only driven the a 5.4 a couple of times and couldn't imagine one in a built out van. Particularly in the mountain west. I have averaged 12.8mpg over the course of my ownership and my van is HEAVY!
I have a built 2wd. 5.4. And drive the highway to hell. ( coquihalla) in British Columbia . No problems in the mountains and get decent mileage still..