Welcome to the site.
I would highly suspect the tail lights. Take them off and see if there is any water in there (that rear window rarely leaks but that doesn't mean it can't). The tail lights have a cheap foam gasket that deteriorates over time--first place I would look.
Did you feel along the wall to see if it was wet above your pillow along the wall (like if the window was leaking)? Since that is at the very rear of the coach I'd suspect that the water is either coming in from the tail lights or coming up from somewhere.
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