A mattress is really your personal choice regarding, price and comfort.
From my experience with RV mattresses I would make sure the new one is not too thick because that sometimes interferes with a drawer or cabinet/closet door that is next to the bed, not being able to open.
Some camper cabovers are narrow and it can be tough to remove the old and in with the new. Sometimes you have to fold it and use ropes or straps to bind it in order to remove or install it. So make sure you can work that out if need be. You might have to fold a new mattress to get it in.