Albertaguy: My heart goes out to you. Prior to purchasing our fist slide unit we read about a similar tragedy. We now alsways do a "Jinny check" (in her memory) and make sure the cats are secure before anything moves.
To the original poster. Go through your unit with a flashlight and check everywhere (under beds, between slides, where closets meet walls, etc) for nooks and crannies where a cat can hide and close them up. It is amazing how a fat cat can squeeze into a tiny place. We "lost" our girl on a departure morning. After scouring the unit and campground for hours looking for her and debating our next move in tears she appeared out of the bedroom as if noithing was wrong. She had found a tiny openning between the bed and the night table that led to a small empty space and hide there. The openning now gets stuffed up as soon as we open that slide.
The hardest part is what to do with that darn litter box. (we keep it in the shower stall (drain plugged of course) and shower door modified with a hook to keep it open when not travellig.
On the more fun side. Our cats come out with us on thier leashes and really enjoy going on walks and meeting people. They are a great ice breaker.