You are probably having to do this the hard way.
The base vehicle before conversion will have come with Fiat Professional Ducato wiring. Depending on the model and specification ordered (short wheel base, long, cab & chassis, flat bed, tipper, etc.) then additional electrical connections etc. may have been included ready for the converter to connect to.
If I were you I would be looking at the camper's consumer / power unit for make and model numbers etc. This (if you can find info on the web/elsewhere/handbook) will tell you what functions the consumer/power unit has. For example some have integrated automatic switching for the fridge and other items. Key here is to get a basic understanding of what you have.
Now the hard part. You have to follow all the cables coming from the consumer/power unit. Taking the cover off will help eliminated by sight those "output" feeds to lights, water pump, 12V sockets etc. The BIG / CHUNKY cables will almost certainly be main 12V power feeds. Follow where these go. They will be taped or in trunking and hopefully not obviously associated with the base vehicles wiring.
If you have a volt meter and are competent in using it then you can find the various permanent and switched 12V wires.