Sure sounds ground, if brake (sig) is looking for ground-(traveling out the dual filament bulb-via tail light wire-common). I assume fixtures are single wire-no ground wire. I dont know how 'gutted' camper is but if walls open running a ground wire is best. Connected to the incoming ground lug.
Grounded thru skin, 40 years ago probably worked ok-. At minimum Id try to get a ground wire to taillights.
If you cant get to marker lights, walls intact, I guess it would be pulling/cleaning ground connections and fixtures. To save grief I'd use a known good fixture to test at each location. More than likley not only bad connection where each light is mounted but probable corrosion in the fixture bulb socket itself (or burnt bulb). Once you have known good +/- at location then address the old fixture or replace.
I do have a sort of unrelated question-but how you described (leaving 12v lights when not on shore) does this camper have AC lights?