Poor Mileage in 6.5L Diesel
 

I have a 96 2500 with the 6.5L diesel. When I got it at 205k, I was getting roughly 16-17 mpg. Lately I've been getting around 11-12 mpg. I changed oil, ATF, fuel filter, checked tire pressure, and I don't even remember what else. I have been running B100 in it, but typically it only decreases mileage by 1-2 mpg. My dad's friend had a Suburban with about 325k on it that he ran the same B100 in and still averaged 15-16 mpg. Any help?

Thanks,
Ross