I do not own a 4500, but since no one else is reponding, I will give feed back on my 3500 pulling a 14,000 lb. fifth wheel.
My truck is also a two wheel drive if the "C" in C4500 means the same thing. My truck weighs close to 8000 lbs. when we have it loaded with equipment for a trip. Not quite 9800 lbs. but getting close. I have never run any chips on the engine, and have over 16,000 miles with most of that towing.
I am very pleased with the ability of the truck to tow my trailer, even out West where we have several significant hills between the West coast and the East. I get just about 10 mpg while towing. Less in the mtns. and more on the flat lands.
My secret is simply take it easy. Wind resistance alone will kill you if you go too fast.
I hope this helps.
Don
2003 GMC 3500 SLE, D/A, EC, LB, 2WD, White/Pewter, DeeZee boards, 5th wheel