Water flows downhill, but capillary action in the carpet can cause it to travel upward. I suggest removing the carpet (it needs replacing anyway) and see if there are any clues underneath it.
You say that the door seal seems OK but it still would be my prime suspect along with the window in the door, especially if it's an openable window. Lay some colored construction paper (like we used in elementary school) on the floor of the step, and simulate rain on different parts of the door, one section at a time, waiting a while before checking the paper. Start at the bottom of the and work your way up.The paper will discolor with moisture and, hopefully the stains will point to the source.
Have someone inside to check the paper so you don't complicate things by opening and closing the door.
Another approach would be to cover the door with plastic, taped to the sidewalls, eliminating it as a culprit and try the construction paper trick during the next rain. If the paper isn't stained, you know the door isn't the culprit. If it's stained you can try successively covering other suspect areas. Since you're in Seattle, rain is frequent so successive tests shouldn't take too long.
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Bob C
2002 Itasca Suncruiser 35U
Workhorse Chassis
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