The answer is: "It depends..."
If your dimensions are close enough to a size that is produced: you might be able to use it...
A lot of RV mattresses are short. (The Queen bed in our rig is 60" x 74".)
You may have luck shopping for RV fitted sheets.
Good Sam Member 843599689
FMCA Member F473304
Current coach: Tiffin Wayfarer 24 BW
"History shows again and again; how Nature points
out the folly of man..."
We have used fitted sheets in all the RVs we have owned. Measure and match to closest size. Like Bob, we have a short mattress in our coach, a short king. I bought the sheets for that on Amazon. It was easy to search on Amazon for sheets that matched our new bed size. In our last class c, the jack knife sofa was a full size while the dinette was a twin size.