That's interesting that they would use the plumbing as the main grounding point unless there is a point where the copper is tied to the trailer body with a positive connection.
The 12 volts system grounds out through the trailer body. Considering the age of the trailer and the fact that things may not be a tight as when built I would make a connection to the frame and one to the trailer body. If from the inside you can get to one of the trailer body frame members that is heavy enough material to make a good ground to and assuming the rivets at tight throughout the body you should have ground throughout the trailer.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles