RV mattress sizes are generally irregular as compared to home mattresses.
We've found that most any sheets large enough can easily be held in place with 8 each tarp clips and some cable ties. Fitted, flat, cheap, expensive... whatever, all will work.
I tie 2 clips together using a cable tie. We then use a set of clips under each corner to hold the sheets snugly in place. We include the bottom end of the top sheet corners with the clamping. That keeps the top sheet in place also.
Works for us even at home because we find the bottom fitted sheets often pop loose with a thicker pillow top mattress.
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