On my previous Sundance, there was no power for a fan so I did what David suggests and wired to a light. Fortunately, those lights had switches on the individual lights, so they could be switched off at night and still use the fan. My current Sundance has led lights. Yours is probably is the same. While adding a switch to the lights is technically possible, it probably wouldn't look too great.
You mentioned you already have wiring in place. Not sure if you have 50 amp service and prep for a second AC. If the answer is yes to both, those wires should have been connected and ready.
This may take some hard investigation to find the problem. Since you say neither the positive nor the ground is connected, I think I would start by disconnecting all the power and looking behind the load/fuse center for an unconnected wire. If you find one, test to make sure it's the right one. Then connecting would be fairly simple. If not, I would start pulling down lights and removing switches from the wall and see if you can find/identify the path of the wire and check any bad splices/connectors. The wall with the light switch and slide out switch would be a good place to look. if you have an endoscope, you can try looking between the ceiling and the roof or in the walls to see if you can tell where the wire runs.
Finally, check the booster for the TV antenna. It has power, so you may find the wire there, or maybe you can run a wire to that location to pick up the 12 volts.
Good Luck.
