I'm going through this right now with my PUP that I'm renovating. The two mattresses that are in it are a weird size: 69 in x 48 in. That's not a standard mattress size, not for at home or for campers.
The foam on our mattresses are awful. They're about 2.5 in thick. I wanted to replace them so I ordered two double/full sized memory foam mattresses and I have to cut them down to size.
For under the mattresses I'm laying down Reflectix, followed by anti-fatigue mats. There are no mattress covers that you can buy to fit our size mattresses, so I'm taking two flat sheets, trimming them down to size, sewing them together, then putting a zipper onto them and they will be our mattress covers. Then it's just a matter of getting a double fitted sheet and mattress clips underneath that will go from side to side of the sheet to keep the edges taut. That will definitely be too big but it's the best we can do given the weird size our mattresses are. I don't want to deal with hospital corners so I won't be using flat sheets. Then we have pillows and a synthetic (not down) duvet. I could use sleeping bags instead but I find cleaning sleeping bags is too much of a hassle. With a sleeping bag you need a large sized washer so I have to go to a laundrymat and deal with that hassle, so it's just way easier for me to take sheets off the bed and wash those at home instead.
This weekend is when we're actually doing our big renovation. For reference, I used this post as an inspiration:
How We Sleep {Comfortably} in Our Pop Up Camper