All gas engines of 2000-2005 era in 3/4+ and 1-ton trucks/SUVs get about 8-10mpg towing. You should budget for 8mpg, maybe 7mpg towing. You might do a little better, maybe up to 10mpg. But you'll be a lot happier if you budget for 8 and get 8-9 vs. 10 and get 8-9. 8.1L or 6.0L, 8mpg towing. Not towing the 8.1L will get more like 10mpg city and 12 highway. The 6.0L is better, 12/15mpg.
Ford Excursion V10 is a very good option. I personally do not like the Excursion as much as Suburban of same years. But many people love theirs. The V10 is similar power to the 8.1L and gets closer to GM 6.0L fuel economy non towing.