A while back someone posted a link to a company that sells custom sheets for campers and big rig drivers. I had them make some sheets for my trailer. The top sheet is sewn to the fitted bottom sheet. That bottom sheet has no elastic but instead it has a big fold over pocket at each end.
Anyway, it works great. The only problem I have is that I like to hang my feet out from under the top sheet until the middle of the night. It's a little tricky, but it can be done.
I can't recall the link to the manufacturer. It took them 3 months to fill my order because they kept losing my paperwork. But I like the sheets.