I don't have a Chevy. but that IS pretty dismal mileage.
You might want to post additional details of your vehicle like engine, gear ratio, weight of tow vehicle, oversized tires, lifts and other non factory mods.
Also details of speeds you are running/maintaining, terrain like hills, mountains, grades and even wind condition.
You do need to realize you ARE NOT going to run the "Indy 500" with this combination, it is extremely long, heavy and is like dragging several tons of cement blocks behind you without wheels..
You may be simply trying to hard to "maintain" too high of speeds and stomping on the gas pedal too hard..
SLOW DOWN, lighten up on the go pedal..
Jack rabbit starts also will make you guzzle fuel, you are not going to win any off the line speed records with this size of trailer..
I get 9.5-10.1 MPG with 7%-8% grades in mountains with 26ft 7K trailer and have done that with 2003 F250 5.4 (lethargic 265 HP 2V), 2006 F250 with 5.4 (3V 300 HP) and 2013 F250 with 6.2 (385 HP) all of those had 3.73 axles..