My 2004 held back 9000# to 9800# without any problems. towing in the same area you will be towing. You know you can add the exhaust brake to the 2005 right? Pac brake and Jacob's are the two you want to look at.
BTW you know with a slush box(auto) you can downshift anytime during the descent right? If your truck starts to pick up speed on the descent use the "snub-brake"
video on snub braking. Yes this is a big truck but it works for ALL vehicles!
A manual trans is the one that you need to pick your gear before you start your descent. The rule is, whatever gear you climb the grade in you drop one gear. If it's a long descent drop two gears. But NEVER shift up or down during the descent! With that said I wonder if down shifting is an issue with a fully synchronized manual transmission?
That rule is for big trucks that are not synchronized.
Don