If there isn't enough give in the foam what will happen is the l spine won't be aligned properly, particularly when lying on the side. Look at the ads for matresses to see this illustrated. From what I've read the foam of your v-berth cusions are old and hard, topped by egg-crate foam which may be too soft, and then memory foam which can't work properly because of the first two.
I'd start with at least 6" of medium density foam topped with at least 2" of memory foam. They memory foam should have as little on top of it as possible or it won't work. I found this out when my wife put a thick matress pad on top of our Tempur-Pedic. It made it as hard as a rock and miserable to try to sleep on.