What are things to check when my 98 Cummins 12v starts using a lot of fuel?

Oil looks good smells good. Oil level staying the same. No signs of leaks visibly following and touching all the fuel lines. Cannot smell any diesel. No black smoke out the tail pipe. Engine still have lots of power. No knocking from engine.

Carefully check the fuel economy by filling the tank and noting the miles. Do the same thing every time you fill up and calculate the fuel economy. I don't know the tank size but 200 miles on a half tank might be around 20 mpg.

This is a 32 gal tank. I was getting 19mi in town and 22mi on the highway. Now I'm getting 12mi in town and not much better than 12mi on the highway.

Could something in the AFC be stuck?

I don't know but I found this trouble shooting guide that might help you. ------------------ http://dodgeram.org/tech/dsl/troubleshooting/12v_trouble.htm