The dt360 is of course has static timing, do you know what RPM's the truck ran at that it came out of?
Whats it running in yours?
The real trick would be to time it right for the RPM it's running at highway speed, and also make sure that the injection pump and injectors are running right.
From what I understand about the dt360 pump so far, the injection duration is on the long side, so there might be a lot of injection past TDC.
Ideally, you'd want your injection to be as quick as possible, not long.
The p7100 should do just that, from what I understand.
I don't see why you couldn't get a lot closer to 20 mpg if it's set up right, a lot of dt360 school buses are getting what your getting, and they're a lot bigger and heavier.