I've noticed some special-made sheets in Flying J's made for truck sleepers - they are a combo of the bottom as well as the top sheet. Simple to make up a bed. In my search of the Internet, I've found a site the offers these sheets. But they also offer sheets for RV's as well. And they even offer those special shaped sheets for the twin beds in Airstream TT'ers. Might just find what you have been looking for.
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