We have an X23B, GVWR 4995 pounds. We tow with a 2014 RAM 1500 Quad Cab with the 3.6L Pentastar V6. This seems like a good combo for us.
We get about 12-13 towing (10.5 with strong headwind) and 22-24 regular driving during the spring/summer/fall running 65mph. Winter is about 20 mpg.
Seems like 9-12 is about the norm for towing trailers assuming the TV and trailer are a good combo. It's not the weight as much as the giant parachute behind you that causes all the load. We got about the same mileage with a 3500 pound trailer as we do with our 5000 pound trailer.
2014 Ram 1500 Quad Cab Big Horn
2015 Jayco X23B