I just use whatever fitted sheets I own. I think they are double size that I use on the dinette and jack knife sofa. (I use a twin top sheet on the dinette.) For the fitted ones: more elastic works best.
On our jack knife sofa, the fitted sheet can't really 'tuck' on the back side, but the elastic holds it just fine.
On the "trailer queen bed" I use king size sheets because I had them. They have elastic all around the fitted sheet. I just make sure I pull the fitted sheet way under. I like the longer drop of the king fitted sheet.
2003 Winnebago Minnie 29B