Has the Avalanche ever been worked hard before? If so, how did it feel doing it? By "hard", I mean pulling a grade in 2nd gear and your foot is to the floor and it can't go any faster? ;)
When you say an "extended trip", I assume that means being on the road for several weeks to maybe even a month or longer?
Can you, as a driver, stand hearing your engine pull 4000 rpms for minutes at a time pulling a long grade? Or gearing down and managing your brakes and speed on a long downhill grade?
I'd be more concerned about the 14 year old truck if all it's done is run down the road and has never towed anything before..
Good luck!
Mitch