when I was looking (last summer), I did the best payback analysis I could, using the current prices at that time. The payback on the Diesel was basically never going to occur, between the $10K premium to purchase the Excursion with a Diesel engine (6.0 or 7.3) over the V10, combined with the gas/diesel pricing at the time. Now fuel prices have changed since then, and Excursion prices may have as well (dunno), so the analysis might have a different result today.
Anyway, between 2 cars, and the enclosed box plus lots of tires, tools, etc - we're at the high end of the weight range for the Excursion (V10 or Diesel), and the V10 still pulls things along quite nicely.
If you are pulling only one car, I wouldn't worry about the V10 at all. (unless you have to tow over the Rockies to get to your local track).