I averaged right around 11.5-12mpg towing a 26' Terry with a slideout that probably weighs about the same as your 31' Airstream, and the aerodynamics of the white box weren't as good either. So you might figure on about the same. That was with a 2001 Excursion that I used to own with the V10.
On a 2001, Ford had not yet fixed the spark plug issue they had where there weren't enough threads in the top of the head to hold the plugs in. If you get this truck, I'd recommend that you check the sparkplugs to make sure they're in snugly and not backing out; I'd do it every time you change the oil. If it spits one out (mine did), the fix can be expensive if you have Ford do it.
On the other hand, call me and I'll fix it for you for 1/2 the price
I bought the Timesert repair kit that allows you to fix it right in the vehicle.
They fixed that problem in 2003. Some guys go 150,000 miles on one below 2003 and never have a problem. Mine blew at 36,348 miles, just outside the bumper to bumper warranty. Gotta love it...
But anyway, keep the plugs snug and she'll pull your Airstream just fine.