Last summer we towed our deckboat from the northern Chicago 'burbs to Branson, MO, about 566 miles.
The dry weight on the boat is about 3,500lb. With gas, trailor and other equipment I estimate that it was close to 5,500lbs.
Hills were tough in that it struggles a bit on steep inclines. Stopping didn't seem to be an issue. As expected gas mileage was horrible. Especially when we were getting into the hilly areas of the Ozarks. THe trip computer was indicating ~10mpg, but I suspect it was around 8 or 9mpg.
It did sag a little bit and on large bumps or bumpy roads, you did get a bit of the "tail wagging the dog", but all in all it made it there and back with no problems. I never once felt like I didn't have control or was near the limit of the vehicle.
Going much higher than that might make me think twice though.