Issues with GMC 6.0 L

I'm using a 09 GMC Sierra 2500HD for towing my 10,000 lb Jayco fifth wheel with the 6.0 and six speed tranny. It's working just fine for me also. In May 2012 we towed the 5ver from Wichita to Albuquerque, to the Grand Canyon, to Las Vegas, to the California coast then home. I only had one area on the way home that was a pretty long 6% uphill grade where I thought it struggled. It was turning close to 4500 rpm and only making 40 mph at the top of that grade.