If you do a search for full size XL sheets, you can find a couple of suppliers that make sheets that fit the long full size mattress used in the 21. They are also sometimes available at RV dealers. I purchased these flannel sheets (although the price seems to float around - I paid $79.00). Soft but fairly heavy sheets - great for cool weather or even hot weather without a blanket.
Trailer: 2017 Escape 21 ("Hotel California" -Aug 2017) Tow is 2016 GMC Canyon Duromax Diesel
I get real hot at night (hot flashes that never stopped) so we sleep on Sheex Original Performance sheets that really seem to help us to sleep cooler. They have a soft silky kind of texture made out of the kind of fabrics that professional athletes use for performance. We have a set of regular full size ( not full xl ) sheets from them and their bottom fitted sheet fits the bed on the 21 real well. Easy to get on and not too big or small.
Lisa & Pete
"What if loving what you have is everything..." - Ellis