[TexasCamper]

I think someone mentioned, but I'll repeat that another reason for the wet curtains/valence is that the window(s) may have been slightly ajar. For the front & back windows (with the stone guards), I've gotten rain into the camper when I didn't close those windows all the way.

For all of the side windows, you are supposed to 'snap' them closed at the center fastener. Sometimes the black rubber gasket along the outside rim of the window frame gets kinked and prevents the window from closing completely. That's fairly common and you'll see a few posts on the topic. I go around the outside with a credit card to help close my window fully (when it is important to do so, otherwise I accept the tiny opening). But a hard rain with wind will get a good amount of rain through the open crack and dampen the curtains.

Since you're a potential buyer, you're entitled to a second look. Take a closer look now that you have some ideas on what it could be. Open & close the window and see if sometimes if the fastener doesn't click and close the window tightly.

John