Here's my experience....

In town, back & forth to work, 13 (at best) - but to qualify that, my trip work is only 3 miles (a tank lasts about 1 month)

We just traveled TX to NC & Back - on the way to NC we towed our 7k# TT and got 12, on the return not towing we got 17 the first 1/2 of the trip and 15 the 2nd 1/2 (30 mph head wind).

I also learned a valuable lesson... when towing, never let the tank get below 1/4 and when the low fuel light comes on... it's to f'ing late! Sat on the side of I10 for an hour waiting on roadside assistance