Bought a 2012 rockwood roo 183. Tow with a 2007 4runner v6. Added Reese Cam WD hitch, transcooler, k&n filter, Trailer weight loaded is around 4600lbs.
4 runner towed more than 2500 miles over the summer and did fine. big hills were the most noticeable but never a big problem.
Towed a 3000lb Coleman Cheyenne for 4 years with same truck and none of mods noted above. Never an issue even towing over the Kagamangus highway in New Hampshire.
Towing the rockwood gets me about 10mpg on average. I have not felt under powered or out of control when towing with this vehicle.
05/19/2012
Added a Oil cooler and a Steering cooler from E-trailers. Both were pretty easy to install and for $160 it's cheap insurance.