11 year old battery? If it drops to 7ishV on the starter the chances are excellent it's the battery but it wouldn't hurt to put an inductive ammeter on the starter cable just to see what it draws. It would suck to put in a new battery and find out it was really the starter. DAMHIK...

If you're looking for a big CCA battery they are as close as your nearest Kenworth type dealer. 1000ish CCA batteries are about 55 lbs so heavier than what you are running but they should be coming on sale for the winter season, $70-$90.

Kevin