Anyone towing 17k GVWR 5er with 2004 GMC Sierra 3500?

I towed a couple different rvs through BC with my 2003. The first one was a 1999 Citation double slide that weighed around 10,000 lbs. It pulled that trailer absolutely great. The next was an 03 Citation triple slide weighing in at 13000 lbs. It pulled it fine but the extra 3000 lbs proved to be too much for it on the down hills. I would put the transmission in low gear and the Duramax would rev to 5000 rpm in order to hold back the weight on 8% grades.