When we had a corner bed, we used a Travasak, now known as a Superbag. It's a queen-size sleeping bag with zippers up both sides and sheets that velcro in, so you can remove them for washing. We still use this even though we now have a walk-around bed!
www.rvsuperbag.com
If you're talented, you can make something like it yourself. Or you might find them on ebay also.
No question that with a corner bed, you two have to get on the same nightime pottie schedule. But we found that doable, given the other benefits of that size motorhome.