I'm glad you have been able to fix the problems you had, and can get away. Don't forget to bill the dealer for your time and materials.
But the missing negative wire raises a question, did you mean the cable was missing or just that it had lost continuity? If it was missing, how on earth (please excuse the pun) did it get through its initial PDI when it was first sold? I know you don't have the answer to that question. I do wonder if some how it was making connection but by using the metallic parts of the caravan, so it might have worked initially, but as the caravan has been used the metal to metal contact of body panels could easily degrade and ultimately over time, both corrosion and physical separation of panel joints reduced their ability to carry the zero volts line for teh 12V lights.
If that is what has happened, then the caravan did not meet NCC standards which calls for all 0V circuits to use wires and not rely on panel interfaces to carry the current.