#6500 dry probably means it was pretty close to being #7500 or more loaded.. :)
That "#8000 tow rating" also does not necessarily mean a fully loaded travel trailer either.. ;)
With all that being said, if this occurred going up the grades, while the computer and the trans are 'good' at selecting the proper gear and such, did you have it in "tow/haul" mode? Did you just mash the gas and let her rip?
Lots of things that 'could have happened', and impossible for anyone to say, but I'd say you just exceeded your trucks limits going up that grade and it let you know.. ;)
Good luck!
Mitch