Our TT has the front queen and then the back double corner bed with a single over top of it. I understand your dilemma. We only make the bed up if grandchildren come. I get them to do it with a fitted bottom. If I have to do it myself I use a flat sheet on the bottom and tuck it in....you still have to reach over,but you are not lifting corners to stretch the fitted sheet on.
The sleeping bag idea is a good one if you like sleeping bags. We like fresh sheets and blanket every 3 days...sheets are easier to clean.