Hi there: I don't know, there, how's that for being helpful. I do know my 66 Trade Wind, my son's 69 Globetrotter, and my wife's 86 Classic all have drain lines exiting the belly pan. They are typically for "low point plumbing drain", or fresh water tank drain, or air conditioner condensate drain. You are holding a copper line which reminds me of the fresh water plumbing system. Our drains are typically hoses.
I assume you can see an open ended copper line under the trailer. Try to trace the line and find out where it goes.
David
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