14mpg hauling a car behind that Limo is pretty darn good!
Hauling an enclosed trailer with a gas pickup/Limo at 70mph typically will give you 8 - 10-ish MPG YMMV. Make it an open trailer and add 3-4 MPG to that. Go to a diesel and you'll get 14+ with an enclosed trailer and 16-21mpg open trailer (but with twice the torque in the diesel, you won't notice that the trailer is back there).
I got about 9-ish mpg with Pearl trailered behind a gas 5.4 liter pickup. I got 16.5 with the same car/trailer with a 6 liter diesel. Same car/trailer behind Chris' 7.3 liter chipped diesel got 21-ish. Both of us saw a dramatic drop in MPG at 80 and above.