Here's a shot from the 1977 Service Manual showing the fresh water tank installation. Can't swear to it being identical, but many items didn't change often in that time period. Click the beige bar (if it's there) over the image to make it bigger.
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That's something I would want to call the previous owner on. At least to get the parts that hold it in...
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“Courage is being scared to death, but saddling up anyway.”
...John Wayne...........................