Might be something wrong with the trailer. Have you greased your bearings recently? Tire pressure good etc? Maybe it just has the aerodynamics of a sea anchor.
I made a trip from CO to CA and back around Christmas. 6200 lb (measured on scales) travel trailer and fully loaded Expy with family, dogs, toys, etc. Cruise set at 72 and usually maintained that speed. Averaged 11.9 mpg over 3,080 miles traveled, hand-calc'd. Those numbers exclude times I drove around without the trailer attached.
Trailer is in great condition, borrowed from a friend who just got a new Denali XL 6.2. so we can soon do side by side comparisons He was formerly a chief Engineer at one of the big three, so trailer is in more than perfect shape. One of my hobbies is Adventure Motor cycle riding and many buddies have a wide range of tow vehicle, but unfortunately a constant theme is poor towing MPG from the Eco Boost