We have been using these Blue Magic Sheet Straps: Bath : Walmart.com
for the corner bed in our Safari. Our local BB&B does not have the straps in the store.
We purchased three sets (4 straps per package). One set of 6 straps holds the mattress pad in place. Another set of 6 holds the bottom sheet. In the rear corner against the wall where there is the large curved mattress edge, we use three of the straps snapped together to form a T. The three point connection keeps the sheets fitting better.
We use standard full size sheet sets like you can buy at any store, not RV sheets, over a 3" topper.