Our bed has complete walls on two sides and about 14" of wall on the other two sides. There is only one corner I can't reach standing on the floor. I crawl over to the far corner with the bottom sheet and tuck that corner snugly under the mattress. Then I get off and tuck in the two corners that just have the shorter walls, pulling the sheet tight. Finally I tuck in the completely open corner. We pretty much just toss the top sheet and blanket over the bed and lightly tuck in the bottom, which has the open corner and one short-wall corner. It's a nuisance but it works.
When we got the MH, I tried to find those "bed-in-a-box" things. The original company had gone out of business, and reviews of the new one were that it wasn't nearly as nice, so I just went the normal sheet route and can live with it.