I've towed a 7x14 enclosed cargo trailer with a 2009 Odyssey and got about 10 MPG between west Texas and Scottsdale AZ, with cruise control set on 62 MPH The trailer grossed around 5000 pounds going and 2000 pounds coming home empty. Weight didn't matter, it got about 10 MPG both ways. It's the aero-dynamic drag that kills MPG. I usually tow at around 62 MPH, but if I speed up to 70 or 75 the MPG goes in the gutter.
I now tow with an F-150 EcoBoost (twin turbo 3.5L V-6). The EcoBoost is a much more capable towing engine because it has so much more torque, but it still gets only around 10 MPG towing that cargo trailer or our small travel trailer that has about the same frontal area.
Grumpy ole man with over 50 years towing experience. Now my heaviest trailer is a 7,000-pound enclosed cargo trailer, RV is a 5,600 pound Skyline Nomad Joey 196S, and my tow vehicle is a 2012 F-150 EcoBoost SuperCrew.