12v Electrical Question

[WeRVoyagers]

Hey all, got another question (electrical) that I am hoping someone can help me with.

I have my cell booster and wifi ranger installed at the back of our 377 with the booster installed under one of the side tables next to the couch. Currently both are powered with AC plugs but I am wanting to use the 12v DC hardwire plugs that came with them (that way even when we are not hooked up to shore power they still work).

I have found two unused 12v positive leads and one dead wire (assuming it was for a switch) in the little locked door outside behind the kitchen slide, which I already have a hole drilled into there for the booster and wifi ranger cable to go through. However, I can not find a ground/negative wire anywhere back there. Does anyone know of a negative/ground wire back in that area? Can I just add another frame ground point back there? Or is that not advised since there is, as far as I understand, a common ground for the whole trailer 12v system. I see one white wire, which seems to be what Jayco uses for ground/neutral in other places for the 12v system but it seems like it goes to the marker lights so I am not sure if that is only neutral/ground when hooked up to the truck.

Any help would be greatly appreciated. Pictures are of the install. Bottom left corner shows the white wire I am talking about (not the AC wire, but the one that goes through the lower hole in the back wall).

[JFlightRisk]

The 12 volt system grounds through the frame, so you can run another wire from the frame location of you choice, to inside , where you want it. Just be sure the frame connection is free of paint, and makes good contact.