Ram towing mileage

So I bought a 6x12 single axle enclosed cargo trailer for general use, and sometimes going to swap meets. I have a 2008 Ram 1500, 5.7L that I tow it with, and it's killing me on gas mileage. I would assume newer models would get at least somewhat better mileage than the 08. The last tow was in April, trailer only about half loaded, running A/C all the way, and it got 8-10 mpg. Do any of you have Rams that you tow a similar trailer with, and if so, what year model, engine, etc. Looking for some type of solution. Thanks in advance for the help.

Dustertogo
What mileage do you get with no trailer? What speeds with and without trailer. What axle ratio? What size tire? Any engine or exhaust modifications?

I have an 08 Ram 1500, 5.7L, 4x4, 3.55 (or3.54?) axle ratio, stock 20 inch wheel and tire size. Mine is averaging 16.5 mpg with assorted driving and freeway speeds with NO load. if I drive 55-60 mph for long drives it has been up to 19 mpg. My current mileage is noticeable low but I know it needs a new oxygen sensor and is due for spark plugs.