Thanks to Rob and Linda I was introduced to the RV Superbag (had never seen them before!)
Today I made my own. First, I lay my cushions down and cut a mattress pad to fit the top dimensions. Using that as a pattern, I cut the fitted full size sheet to fit and sewed the 2 together. Then I sewed the full flat sheet to the first 2 on just 2 sides (the end and side that will be against camper wall.) I left the top sheet the full size, but sewed the wall edge flush with the edge of the bottom sheet/pad. This gives plenty of top sheet to move around in, and also will extend off the side to cover the space under the bed. (I hope
I did not connect the comforter to the sheets-----I'll see how it all works. If I decide it needs to be connected, I'll use grommets down the wall side of the comforter and sheets and tie them together. Don't want to mess with sewing on velcro.