I tow a 230DS with a 06 Xterra, which shares the same drive train as the V6 Pathfinder and Frontier and can not stop bragging about the truck!
Mileage for me ranges between 10.5 and 8, depending on load/terrain/wind direction, and I here the 5.6 is comparable. The big issue is not necessarily the weight, but wind resistance.
Using the 5.6 with the 189 will be a great combo, you will not be taxing that at all and I would bet you will get 12 - 10 mpg. Let us know...
2006 Nissan Xterra
B&M Trans Cooler
2011 Nomad 206
Panasonic/Pioneer/Kicker ~ Audio/Video System
Days Camped /2009 / 33