Just got back from a trip all over Colorado, Idaho, Montana, Wyoming and Utah. 2800 miles in 2 weeks in our 2007 V8. I pulled a 23' Airstream that weighed about 5700lbs dry, but was almost always loaded with full water (extra 250lbs or so) and some grey water though I often emptied this, and full propane (adding 60lbs to the tounge weight. It was also loaded down with gear and food.
In general I would say the trailer probably weighed about 6300lbs - but that is just an estimate.
Most of the driving was in the mountains between 5-10k feet elevation. The t-reg did great overall. I'm not sure I would pull that much weight up Teton again with 10% grade. It was working very hard to pull it up, and I had to let it downshift a few times to keep from crawling up. But beyond that, it pulled like a dream and I blew by a lot of rigs being pulled by bigger vehicles. I got somewhere around 12mpgs on the trip average, but that is from the trip computer and probably not very accurate.
Thinking of trading it in for a TDI soon. Would like the improved torque and better gas mileage for daily driving. But I can't complain about the V8.