Joan (jjnj),
Here's another 2 cents of mine. Could it be that with the slide "in" the threshold/support plates underneath the slide (3 white pans with lipped edges) are tilted inward, whereas when the slide is "out" the plates are either level or tilted outward? On my unit these threshold plates/pans collect water that comes from above. They should all three be welded together at their lipped seams and waterproofed (caulked?). They should allow the collected water to drain to the outside. If their tilt is wrong when the slide is "in" and/or if they can leak water at their seams you/we may have at least part of the answer. This is what was happening with my unit. Dig around under the slide out (you may want to remove the couch's fabric covered skirt board), looking and feeling for water in the pans.
The source of my water leak when the slide was "in", as noted in one of my posts, was the diagonal inside seam of the slides perimeter trim. Other possible sources are too many to even think of listing.
As always, good luck.