Your trailer probably had 3 electrical systems: 1. Tow wiring 2. 12 volt system (inside wiring) 3. 110 system which is the plug in connection.
The tow wiring runs off the 6 plug you mentioned. It is better to replace that with the more modern 7 way. The charge line from the vehicle (labeled black on a 7 way) sometimes will operate a 12 volt ceiling light.
The 12 volt system could be only the ceiling light. Sometimes, a battery is added and a fuse block to run other 12 volt items.
The 110 system has either a "motorbase" inlet (like a pronged plug only inset) or a plug in cord. It usually runs 2 interior lights and a few recepticles. The 110 system is separate from the tow plug or 12 volt wiring. Look for a breaker box and trace the incoming wire down and out. You will find the inlet on the side of the trailer or a cord in a compartment.
Please feel free to PM me if you need more information. I very much like chatting electrical.
THX for the replies guys, I located the vehicle that was used to tow it and traced the tow recepticle and have the brake,turn and tailights working just not the running lights. I would have figured they would have been on the same circuit as the taillights but if they are then all the bulbs are bad. Will check that today. I used the red.green and black so I'm guessing the orange would be the 12v interior and/or charging line you mentioned. I'll check out the diagrams here www.myrv.us/electric/ and get back with my progress.
I also managed to fish out the 3 prong 110v external cord and will be testing that in a few minutes.