Depends on what you want/need in a mattress. Some of us need them firm (I do best with a 2-inch fiber pad on a hard surface) and others want no pressure points but don't mind sag. You need to shop mattresses by trying them, find what meets your preferences.
I lucked out, my RV came with the same hotel-grade, unpadded extra firm innerspring mattress I've always preferred. Put a two inch foam topper on that, the back pain would cripple me. My baby sister needs the foam pad.