Help don't know which way to go to fix truck!

[Waterloo]

Well, sounds like your compressor knocking. I had same issue on my 871. Cummins has a head kit, no need to replace entire compressor. Install that and clean the soot out of the lines. The fuel mileage is more than likely winter blend fuel, my mpg is running the same as yours, I normally lose 1 to 1.5 mpg in the winter, this year is no different.

If you have not done the EGR tuneup and still have the mandate, do that ASAP. Also, in regards to knocking, have you replaced your crank shaft damper? Cummins recommends replacing every 500,000 miles. I would do that, when they fail it can get very ugly and expensive. If you replace, paint it, or no warranty.

Also, have you done a recent overhead? I would do that too if it has been awhile. I would also drop the oil pan and inspect the lower end. It is time to do that too. 860,000 miles is getting close to in frame time... I would also check the coolant system for any excess pressure build up. I would recommend going to NAPA and picking up a block test kit. https://www.napaonline.com/p/BK_7001006

Good luck, been there and done that.