When we had our pop-up, we never found sheets that actually worked on both bunk-end beds. I was fortunate, my mother is a seamstress (I can't even sow a worm on a fishing hook!)...
We purchased 2 flat sheets large enough to cover the mattress. My mother sewed them together like a pillow case. She also attached some strings on both sides so they could be tied together.
Because this was a Pop-Up, getting the mattress off the bed was extremely easy. We simply put the mattress in it's new "pillow case", tied up the end and put it in place. Worked great! We used sleeping bags on top of that for years. Eventually, we moved to a real sheet and blankets, and then we discovered ELECTRIC blankets! OH MY! Wonderful on chilly nights!
Actually, when we got our first TT and then our second TT, my mother sewed up more "pillow cases" covers for all the cushions, bed mattresses, and everything for us. A couple times a season, we'd remove them, and washing them in the washing machine. For us it worked wonderful!
Current camper has a real household mattress and we now have real furniture, so no need to do this any more for us. But, if you can't find the item, you make it yourself! That was our solution.