We have a queen corner bed in our MH and it was hard for the wife to put on the bottom sheet so I took over that job. We had a new mattress made and it is too tight to get the fitted sheet on all the way. I start at the back and don’t be on the sheet you are trying to put on, push it down and around the back wall and corners as far as I can, then take 3 big safety pin and pin it to the edge of the mattress in the middle and each end side, then back off and tuck the sheet along the side as backing and finally get off the mattress and pull it tight at the bottom and at each corner and the middle again use big safety pins attached to the mattress.
We use each use a modified RV sleep sack for our top cover. I tried using a full RV sleep sack (or as it now called a RV Superbag) but it kept bunching up under me when ever I turned or got in & out of bed. This solved our problem. Regards, Dick