If you cannot find any access into the suspect area, you might consider renting a "bore-scope" normally used by gunsmiths to inspect the bore of a gun. You may drill a small hole near the area, thread the bore-scope into the hole and inspect the pipes for seepage. If/when you locate the leak, you will need to remove a panel for repairs. This will also remove the hole you drilled. When you replace the paneling with a new one it will appear original. OR, you might hang a picture over the tiny hole(s).
That's my thoughts.
2000 Winnebago Ultimate Freedom USQ40JD,
ISC 8.3 Cummins 350, Spartan MM Chassis. USA IN 1SG 11B5MX,Infantry retired;Good Sam Life member,FMCA." My fellow Americans, ask not what your country can do for you, ask what you can do for your country. John F. Kennedy