We have a two-berth caravan and for ease of storage bought two memory foam mattress overlays, which we place side by side when making up the double bed. They store very easily under the seats, by rolling up.
It is recommended to get it no less than 2" deep if you are of heavy build. We got the 2" deep anyway as I have had a disc out and found the caravan seats far too firm. Now it's a blissful nights sleep in comfort.