We keep the dinette "bed" in the Lil Bigfoot
made all of the time now. I use an eggcrate mattress topper (double bed size), which we trimmed to fit the round corners of the trailer.
I plan to use a mattress pad to create a zipper cover for this, but in the meantime keep it covered with two flannel double bottom sheets. These just go onto and around the topper, they are not tucked down around the dinette cushions. (I tried that first; it was impossible and thankless) The friction between the foam and the flannel also seems to keep the sheets well in place.
But I do tuck the top sheet and blanket and/or quilt down between the cushion and the trailer around the foot of the bed, next to the sink area, and partway up the other side (leaving enough room for the late-to-bed partner to get the covers back and get IN).
The tension between the cushion and the carpeted side of the trailer does a great job of keeping the top covers tucked in, so this is pretty easy and has worked really well.
I'm using a double bed comforter on this right now, but it is too big (IMHO) and I think a twin comforter would fit this space just right.
That's my bedtime story, and I'm sticking to it.