Since you are doing 4x4, I'd go with the V10. It'll have more torque and better gearing for if you leave pavement, or end up towing heavier in the future. And will me much easier to sell if you ever decide to change things up.
The 5.4L advantage gets narrower as profile goes up, since most of the load is from pushing wind. I think the reason you see a huge delta on fuelly is simply that the 5.4L was biased towards van bodies and the V10 is biased towards RVs and box trucks. In fact, with the 6-speed behind the V10, it might get the same fuel economy as the 5.4L with the 5-speed.
You'll probably 11 with the V10, and maybe 12 with the 5.4L
Another advantage of the V10 is the nearly silent idle, in case you need supplementary power or heat.
