I have a 2012 RAM 1500 with the 5.7L Hemi and 3.92 gears that is used to pull a converted V-Nose cargo trailer of comparable size and loaded weight of the mid size Casita TT's
On a month long trip this past June around the Cariboo central interior and mid west coast (Bella Coola) in BC Canada, the truck averaged 13.8 miles to the gallon overall.
The roads ranged from flatish to low mountains (for BC anyway) to the steepest grades of the Duffey Lake RD (steepest paved highway in BC) and Highway 20 ascending up the long, wet, slippery and infamous Heckman Pass (15% grades) out of Bella Coola where gas mileage hit an all time low of 9.2 miles per gallon.
You may not ever be traveling in this type of terrain, but even at only 3,000 lbs or so, a V8 engine is very useful as compared to a V6.
On the highway and on a good non towing day, the truck can get 22-23 mpg if I keep the foot out of the gas. Not great but still acceptable.
You may want to also look at the 2014 RAM trucks with the Eco-diesel when they are out on the dealer lots as I know I will surely be taking one for a test drive.