Builderdad, I cannot tell you which one is the main pipe but there is another forum on the internet with people who can tell you.

Here is a link that might be useful: Mobile Home Repair Forum

I worked at the fairmont plant in Nappanee,In. from 1990 to 1994 not in the single wide plant they had different plants for the different models. As to how they did the plumbing they did have prints for each model but deviated a lot from the print when other people did things in their area that caused a conflict not really much help but just thought I'd throw it out there

Normally there is one place where the water line hooks into the house. From there you should be able to trace the water lines to their final destinations. The same is true of the electrical. The electrical comes from the meter to the house and from there to the panel. You should be able to trace most of this from the entry points to kitchen and bathrooms. Did you demo all the inside electrical or leave it in place? The waste disposable system depends on the house. Most single wides have lines running from kitchens and bathroomsn to one larger line that goes the length of the house and has a connection to the septic or city sewer system either at the end or the center backside of the house. Bear in mind that a lot of this can be modified depending on the needs of the final location of the house. I would get a creeper (if sitting on concrete) and go underneath and diagram what you have.