I had occasion to drive 400 miles this weekend, pick up a 4x8 U-Haul utility trailer and drive back. A U-Haul is less aerodynamic than a tear, but it was about the same size and loaded weight. We got 33 MPG without the trailer and 22 MPG with it. Two thoughts -- I'm surprised that the mileage was that good without the trailer, and it was notable how much the mileage dropped with towing. I know a lot of people towing larger travel trailers who would be satisfied with 22 MPG.
This consisted of about 300 miles each way of pretty flat Interstate combined with 100 miles going over the Oregon coast range of low mountains with gentle grades.