Bed linens for Class B's are a challenge.
We went for several years with two flat sheets sewn together at the bottom, over a mattress pad and with a blanket.
What has worked best for years now is a comfy, washable mattress pad on the bottom, and an all season down comforter with washable cover on top.
Skip sheets altogether.
If you can find a mattress cover with elastic bands at the corner, just cut the bands off. If not, trim off the fabric and elastic that would attach to a conventional mattress....voila!
Wash the cover and mattress pad weekly, as you would your sheets.
Works great, easy to store, eliminates the frustration of trying to fit sheets to something that is not a mattress.