We had the same problem when we first got our 25' FB Twin. The sheets wouldn't stay on the mattress. Somewhere on this forum, I found someone who suggested using a pair of suspenders to hold the sheet in place. I went to Walmart, found a pair of heavy duty suspenders, adjusted them to be as short as possible and clamped them to the edges of the sheet in four places. The suspenders make a perfect X and seem to do the job nicely.
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