It can be hard to find fitted sheets that work well with the bed sizes in many RVs. Here is one method the trailer owner might use to reduce the hassle.
First, buy 8 of these Keeper EasyKlips (Home Depot sells a 5-pack, so get 2 packs). Keeper EasyKlip Tarp Anchor (5-Pack)-89521-5 - The Home Depot
Get some light
, thin rope and cut 4 pieces about a foot long each. Tie a clip onto each end of each rope piece. Cut some of the common non-skid shelf liner (also sold
for rugs) so you have 4 squares, perhaps 1'x1'. Attach a shelf liner square to one of the clips on your first clip-rope assembly. Repeat for the other three.
You can either use an oversize fitted sheet or, perhaps even better, a nice big flat sheet. If the latter, lay the sheet over the cushions and fold the overlap into a "hospital corner" or "military corner" at the first corner.
3 Ways to Make a Hospital Corner - wikiHow
Then, use an EasyKlip to hold it together. Take the other end with its clipped shelf liner and shove it under the mattress, preferably where the weight
of your body will help hold it in place.
Repeat for the other 3 corners. You should now have a reasonably well-fitting bottom sheet, no matter the bed size. The same system can be used to keep the foot of a top sheet in place, if that has been a problem for you.
We had a waterbed some years back, and I'd found something similar (ready-made) to help hold the bottom sheet in place. It worked well. It actually attached to the sheet with something akin to this Coghlan's tarp holder that Floyd linked to recently... Tarp Holder | Repair | Coghlan's
...but I think the EasyKlips will hold even better, and I think they're cheaper.