Water stain on interior wall - pipes in wall? '73 Sovereign

[Shapmaui]

I haven't used my 31' 1973 Sovereign trailer in about 5 years. Getting the itch a couple years ago, I had it's systems tested at an RV shop - pre-covid, getting it ready for a summer trip to Maine in 2020. Well never made it to Maine, and the test failed for the water system, as the shop reported water leaks in the wall. I got back to Maine a few weeks ago.

On inspection I noticed a dark stain on the interior skin just to the right of the window located above the single bed. This is a rear bath model.

Are there original water pipes located in the walls??

I've never had any leaks in the trailer, and I've owned it for 10+ years. Again this stain is about a foot long, (I dont have any photos) and made the original skin dark, and its to the right of the window, not seemingly under anything that could leak.

Does anyone out there know of any mechanical diagram designs for this original AS?

Thank you!!

[SeeMore]

There are no water lines in the walls. Check the gaskets around the window for leaks. If that is not the source, then check for water entry further up the wall and on the roof from overlapping seams in the skin or around through penetrations.