I pull a funfinder 189FDS with a 2006 Frontier, V6, 4WD, CC, six speed manual. Using a BlueOx swaypro WD hitch. I've towed it about 10K miles in all sorts of weather and from sea level to over 12K feet in Colorado. The funfider weighs just under 4K lbs ready to pull out of the driveway. This combo tows nicely. Gas mileage has averaged right at 12mpg overall for towing miles. Low in the mid nine mpg range when slamming into a headwind all day, but those days tend to be rare in the overall season of towing.
I also regularly tow a two horse trailer with this truck, that weighs around 4500 pounds ( depending on the load for the day ) and the truck pulls that fine also. Actually gets better mileage pulling the horse trailer, probably because it sits lower to the ground and is more rounded on the front and side edges, compared to the camper.
Just a guess, but I suspect a Frontier with a manual gearbox makes for a better towing platform, because the gearing is more favorable for pulling ( six speed manual vs five speed auto, 3.69 gears with the manual, and 3.34 for the auto trans trucks ). But still, the factory tow weight limit is the same for AT and MT....so take my comment with a grain of salt.