On our 1975 20ft Argosy MH, even though we have 'made to shape' bedding from AB Lifestyles, and started with lightweight sleeping bags, we pretty quickly change to
1) Get a regular full sheet, folding it in half length ways
2) Sew the seem about half way up the side
3) Measure 18" up the bottom
4) Sew across the measured line
5) Cut the lower 18" off
6) Turn inside-out and you have a perfect 'sheet' sleeping bag.
Add a twin blanket and you are set. In the morning we fold the sheet sleeping bag and insert in the pillow case sham (with the pillow), fold the blanket insert in the second sham, this leave the AB Lifestyles covers for day use (our twins double as a dinette).