Speaking of Fitted Sheets…
I went one step farther.
The Climashield quilt I made for backpacking works so well, I made one for the van. I wanted the bottom end to tuck in tight to keep MrNomer from tossing it, but tucking a quilt normally raises the foot end of a thin mattress. My solution was to sew about a foot of sheet fabric to the bottom of the quilt. This gets tucked in and fastened to the regular sheet and bottom sheet. Everything stays in place and the bed stays flat.