Referring to the first pic in your original post. At the bottom of the pic, I see two wires coming from the battery post - a heavy black one and a red one. The heavy black one almost certainly runs to the second battery box, where it would be connected to the positive post of a second battery if one were installed. At this point it should be connected to nothing, and insulated from unintended contact with anything else.
The red one goes to a black plastic fuse holder. It is barely visible in the photo, outside the box and hidden under the green wire. Although it is a little hard to see in the photo, the other end of this fuse holder probably has a small black wire coming out of it. This wire carries power into the TM. If you open up the fuse holder (push and turn), you may find that it contains a 30-amp glass tubular fuse, that is either blown, corroded, or entirely missing. As a short-term fix, you can replace this fuse - any hardware store - and it will give you power back into your TM. Long term, you should replace the entire fuse holder with an automotive blade-style fuse holder. See
https://www.trailmanorowners.com/for...&pictureid=760
I can't spot where the power to the jack peels off, but it is somewhere in the rat's nest. But since the jack works, the fuse I mentioned may be good, and the problem lies farther down in the rat's nest.
If we need to pursue this further, you will have to pull the wires out of the boxes so we can see where they go, and what is connected to what. If you find another fuse holder in there, be sure to let us know.
This should restore power to your TM. If it does, we can explore the rat's nest of wires in the empty box. The connections are simple, and aside from the long messy wires, easy to understand and easy to connect. In the meantime, make sure that none of the bare wire ends in that empty box are contacting any other wires.
I doubt that the schematic will be of much help, but it can't hurt.
Bill