If you cannot find any obvious intrusion from the roof down, how about from the floor up. That may sound ***f*r*e*e*k*y*** but is it possible the wet spot came up through the floor? You know water splashing up from the road? You said it was raining.
I had a very strange event happen to my Outback. Don't know when it happened, but the slide out closet in the bed room had water on the bottom. Clothes were wet at the bottom where they touched the carpeted bottom. No clue when or how or where that happened. Dried it out, and we've never a wet spot in there again. I'm beginning to think my wife might have put a cup of liquid in there and it spilled when traveling, who knows. Sometimes things just happen?
How about the refrigerator? Did it drain inside the camper? Stranger things have happened. I thought of this possibility when our floors occasionally get wet because we forget to dump out the dog water bowl when on the road! These are just some ideas to think about .... beyond the roof! One more thought, where is the air-conditioner located with respect to the wet spot? It's possible condensation may have dripped on the floor.
If you figure it out, let us know. Inquiring minds want to know! Good luck in your search.