Well, there are several factors at work here:
1. The slower you go, the less horsepower it takes. So the smartass answer is cruise at zero kts and you will burn zero fuel. But you didn't want that answer.
2. The power required is exponential with speed, ie it takes more than four times the power to go 8 kts than 4 kts.
3. The fuel consumption of diesel is related to the power produced, from about 18 hp per gph at 2,600 rpm WOT for that engine to about 15 hp/gph at 1,400 rpm.
4. Diesels need to run at a minimum horsepower to heat up the engine to maintain operating temperatures. Any lower and bad things happen.
So all in all I wouldn't run that Cummins any lower than 1,400 rpm which means it is producing about 40 hp. That much hp will push you to about 6.5 kts and you will burn 2.5 to 3 gph. You might want to kick the engine up to 2,400 rpm for about ten minutes at the end of a day cruising at that speed. That will burn off any carbon that builds up on the rings and injectors.