I think theres two ways of doing this like you said:
1.)Create a DC system with the rectifier using the red and black wires with capacitor.(they were designed to charge a battery)
2.)Create a DC system using the red/yellow wire, and ground to frame.
I'm a mechanical engineer and circuits was a dreadful class for me. I was speaking to my buddy who's in electronics and it seems like we couldn't find an issue with this until I read the trail tech paperwork. I wanted to use the red/yellow wire as + to accessories, and ground the black to frame, with the AC ground. The documents say not to though.
EDIT: Is it because grounding AC to frame, and grounding trailtech DC to frame could cause AC to go up DC ground?