We have the 4" 'memory foam' mattress from WalMart (cut to fit the curves) in our 2001 Casita
17SD. It came with a 'fitted' cover which we modified with large safety pins so it stays tight. We use a fitted microfiber sheet on bottom (works great, just a little difficult to get the corners right but after initial 'making' it's great). A flat microfiber sheet on top followed by an unzipped' thin fleece sleeping bag, does fine for us 90% of the time. We haven't yet spent a winter night there but since we generally stay in CGs with electrical
hook ups a small 'cube' heater provides adequate heat. If we need more cover we add another thin fleece bag, & since we leave the rear dinette 'made-up' all the time as our bed, making the bed each morning just involves pulling everything tight & tucking under. The small 2-person side dinette serves us well for eating, sitting, reading, etc. We love not having to make, set-up the dinette every morning.