If you have ever seen fifth wheels built you would see that it is nigh impossible to find a 12 volt wire in the wall. I don't think manufactures even supply a wiring diagram that would be helpful in your case.
My recommendation would to be to splice into the wires going to a light fixture. Hopefully you can find a light that you can remove and get to the wires so you can splice in and run your new wires hidden under the ceiling or behind wall paneling or cover your wires with a wire cover glued or screwed to the ceiling.
I installed some 12 volt plugs in our living area by splicing into some 12 volt wires that supply the refrigertor. I managed to fish the wires up into the wall next to the refrigerator. Turned out real good.