Paul, looking at a picture of your galley is it the ceiling directly in front of cabinets or the wall surface above the mirrors?
If its the wall under cabinet, is the wall paper inside of cabinet have the same type surface as you discribe outside under the cabinet?
Can you see the seam at top of slide on outside of slide, exposed to weather, are these sealed good. It would be the outside edge and upper outside ceiling edge of slide.
The reason I ask, some people have had the upper edge of slide that closes against your inside wall not properly sealed, if this was the case your ceiling over cabinets should have a stain. If its the wall of slide check the top edge of slide of outside wall. When you cook on stove you have your fan on to remove moisture from boiling pans, so that moisture, shouldn't efect the wall paper.
You have a nice coach lets try to keep it that way.