I don't think there is a no contact 12 volt voltage probe. I seem to remember seeing one a few years ago but now I can't find one. I bet it didn't work well and was discontinued. Even the 120 V AC ones aren't perfect and their signal must be hundreds of times stronger than for 12VDC. Sorry, I can't remember the colors. It is possible to stick a sewing needle into the insulated wire to check for voltage.
I like the idea of using the hitch wiring for an outside camera. Your rig should have the wiring even if you have no hitch. On my Ford, the hitch wiring goes back to where the end of a van would be, some distance from the end of the MH. There is a connector there; easy to stick a multimeter probe into. Mine has a plug into the connector to supply power to the house batteries. I would solder onto the wire rather than buying a plug to fit the connector.
On the other hand, a new wire from the fuse box could go through a switch in the cab so you could conveniently switch the camera on/off. I do that a lot - my camera is inside, looking out the back window and hooked up to the house DC panel.
