I don't know if this will help or not but it's worth a try. Beginning on page 272 of the Freightliner Sprinter Operator Manual is the section on fuses. There are 3 fuse blucks, one under the steering wheel, one in the battery compartment and one under the driver's seat next to the door. The block under the driver's seat may
have the fuses that provide power to the wiring harness for the trailer.
I am finding lots of good information in the owner's manual (wish I had started looking there earlier). Of course, it is no help for the wiring in the living quarters that I know of....