We had a corner bed in our very first TT. We took two flat sheets and sewed them up together like a gigantic pillow case, leaving one end open. We then put the sheet on the mattress. Yes, we had to pull it clear out of the corner to do that.
Once on, it stayed on for the season. We used unzipped sleeping bags for about 2 years then. After sleeping, we'd just roll the sleeping bags up and presto! Bed was done, look neat.
Fastforward to the 3rd TT and we don't bother making the bed now! Why? It's just going to get slept in again in a few hours?