Could it be condensation running down the windows and then pooling at a low point? I'm thinking about the cushions on seats right under the windows. I don't have any ideas about under the sink.
We stored our MH in Vancouver, WA last winter and I'll never store it there in the winter again. The condensation in the MH was terrible. There were no roof leaks. People talk about not buying RVs from the Florida area, I don't think I would ever buy one out of eastern OR or WA.
In Nov we're flying down to move it from Vancouver to the Phoenix area for Nov, Dec, & Jan before heading east in Feb. As part of our trip planning we see what major areas we can store it in which Alaska Airlines flies in and out of.
Bill