I tow a 23' trailer behind a 6" lifted Chevy Silverado 1500. 35" tires, stock 3.42 gears, exhaust, intake, and Superchips Cortex programmer. Dry weight of the trailer is 4735 lbs. With the TOW tune programmed, 91 octane fuel, fully loaded trailer, and a quad in the truck, I was able to squeeze out 12.3 mpg on a round trip from Boise, ID to Newport, OR. And that was doing 70+ mph where allowed.
From the sound of it, what you are looking at, a 1/2 ton will do. My Chevy is rated at 10,000 lbs towing with a weight distributing hitch. Steep inclines absolutly suck ass and I can muster about 45 mph up a 7 degree incline. I would say get a 1/2 ton. However, if you ever decide to go bigger in trailer size, having a diesel will pretty much open you up to tow any size trailer. This is where I am at now. I love my trailer but now I'm looking at a toy-hauler. The fifth-wheel tow haulers are too big for my 1/2 ton, and the trailer tow haulers are right at the max I want to go with my 1/2 ton. If I would have purchased the diesel up front, all this would be a non-issue.
Hope this helps