Our last two trips have been up winding and steep forest service roads. I have a 2010 GMC diesel and ran it some in 4Hi. I kept it in tow/haul mode. Our toyhauler is about 28' long. I left our friction bar on and I don't think it hindered anything, but I can't say whether it "helped" much either.
I think the most important thing is to take your time and watch your speed. For a majority of the drive on these difficult roads, I doubt if I ever went 35 mph, more like 15-20 mph because of the blind turns. Of course, it was raining at one point, which added to the difficulty. I was more concerned about the washboard condition of the roads, which caused two of our light fixture covers to pop out during the bumpy ride.
2011 - 4 trips - 12 nights camping
2012 - 5 trips - 16 nights camping
2013 - 5 trips - 16 nights camping
2014 - 2 trips - 6 nights camping (so far!)