Curt,
I've been in your situations way more than I care to admit. I actually kind-a like this sort of stuff. I like taking someone elses screw-ups and re-engineering them so they are cleaner and more efficient. Here's my thoughts on your issues.
1. Unravel all the coiled up wiring.
2. Remove the plug from the rear of the coach
3. Cut the unraveled bunch of wires off to the appropriate length that will easily fit back into the back of the trailer plug without strain on any of the wires.
4. Using your test light, (after you get it working for sure), start finding each and every circuit at the newly cut off ends.
5. That is, turn on the left turn/brake and mark it. Then the right turn/brake, mark it, etc. etc. etc.
Now, you can re-wire the trailer plug. If it doesn't have a wiring diagram on the trap door of the plug as some do, then you can get the schematic of where to put each circuit on line or at any number of sources. Once that's done, re-check all the circuits to make sure that, you've not cross wired them. Don't ask me how I know about that...
Once all circuits check out, then you move on to the umbilical cord. You plug it in to your newly wired trailer plug on your coach. And, then check each circuit at the other end of the umbilical to see what's what in terms of where they are on the plug and, if they correspond to the typical schematics for your particular plug.
Once that's done and you're sure the system is working correctly all the way to the end of the umbilical, then comes the toad. From this point on, you should be able to handle it. Good luck. And by the way, while I'm not up on each and every vehicle made, especially motor homes, I'd bet my house that yours is NEGATIVE grounded. And, that simply means that you need to make sure that where you attach the alligator clip end of your test light is good and clean and a good source for your ground.
Once that's done, they you're off and running for the rest of your tests.
Scott