It is not at all unusual for a leak to develop in a 1975 trailer, for many reasons.
The plumbing may have been stressed in times past due to poor winterization, vibration from road trips, a thin spot in the wall of piping during manufacturing, etc. The possibilities are endless.
I too recommend installing Pex type tubing whenever you do a repair. Either replacing all of it at once, or in sections as needed(my personal choice). Pex stands up to vibration and freezing much better than the copper that came with the trailer in 1975.
"Wimpy" 1/2 ton 2002 GMC Sierra 4X4 Z-71 Gasser
2000 Safari SS 25'