On Sunlines, white is generally the ground wire for the 12vdc systems. There should be at least one fairly large white wire coming out of the converter and going to ground which should be the vehicle frame. When I say large, I mean something around 10 gauge wire. There should also be a large white wire going from the battery location to ground and to the converter. Chassis ground will be some kind of lug/connector that is fastened to the frame with a screw and has a place for the wires to be held by a set screw. It might look like this:
The white ground wire out of the converter probably is routed down through the floor and on to a grounding lug. Should be fairly easy to find underneath the trailer.
You may want to take a VOM meter and check to see if there is 12vdc at the fuses. If there is, then check to see if there is voltage at the light fixtures.
Also, make sure the hot battery wire is taped off so that it can not accidentally ground out.
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'12 F250 4x4 Super Duty PowerStroke 6.7 diesel
2011 to present: '11 Cougar 326MKS
1999 to 2011: '99 Sunline T-2453
SUN264 * Amateur Radio kd2iat monitoring 146.52