Are the bunk beds an unusual size? We have bunks in our 2010 Fleetwood Encounter and twin xl sheets sheets fit fine. Many off the shelf sheets (Target, Kohls, Walmart) are made with extra elastic to fit thick\thin mattresses and that seems to keep the fitted sheet tight enough. Plus its alot cheaper or less time consuming than buying specialty or custom making your own. Even if they were a little loose I don't think my kids would care.