The owners manual for the boat or for the Cummins?
The boat owners manual wouldn't have any Cummins info. in it. You would require the Service Manual for the specific Cummins model you have. Those don't come with the boat and need to be purchased separately.
I'm not completely sure how helpful the boat manual is either. All the moving parts (pumps, AC, fridge, stove, etc) are 3rd party and each comes with its own manual.
The boat manual does address some wire routing, but a few hours, a six pack and a game of 'hide and seek' will yield most of that info. for you. Pick a system, open hatches and start following hoses, lines and cables.