It should be a blue 12ga wire with silver lettering behind the panel you removed in that upper compartment. The battery end of the blue wire should be loose in the battery compartment near the PD converter, and you can confirm the lettering when you find it. The white wire that of the same gauge/type is the negative and terminates at ground block already.
Another option is to go down the fridge vent.
You are on the right track, and that is a very nice camper... I have done a couple of solar installs on 830's so let me know if you need more help.