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I'm no expert on Argosys, but the 70's era trailers were all put to gether roughly on the same pattern so I will offer you some general advice.
Your fresh water inlet will lead into the frame, but will come up into the interior of the trailer under the cabinetry in the bathroom, almost immediately. So before worrying about dropping the bellypan, open up the bathroom cabinets, and look for a tangle of plumbing in the streetside rear corner. If you can't identify anything leaking there, then yes, the leak may be in that ~12" of line between the bumper trunk and where it enters goes through the floor. You might try removing the aluminum baffle in the bumper trunk that separates the trunk compartment from the interior of the trailer frame. Once that is done, you should be able to see the water line and identify where it is leaking. Of course, at this point you may still need to drop/cut the belly pan to do the repair.