My taillight wires run inside the wall form the fixtures, down and out the bottom edge of the wall, under the bumper. That's where I would look to start tracing....
BUT FIRST, are you sure the fixtures are not corroded, bad bulbs, etc...?
There have been several posts here re. the fixtures they are using on slightly newer coaches that have a poorly designed gasket seal and they leak.... leading to rotten walls. In my case there wasn't any significant corrosion yet, but it had been wet enough long enough to rot wood.... so I can imagine corrosion and bad connections are a real possibility.
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