Check with www.ablifestyles.net
They make RV sized sheet sets, mattress pads, etc. I believe they will also custom make if they don't already make your size. We bought our sheets from them, very reasonably priced, we have been happy with ours.
You can always find stays and adapters to make residential sizes stay on, but we prefer sheets made to fit. It was that way in the V-birth on our sail boat too.