There is some storage space under the berth in the master cabin. ECHO had a queen size mattress that was very heavy and awkward to move out of the way in order to access the hatch under the bed. It took the two of us to lift out and snake out of the cabin and into the hall.
We made our new mattress in two sections. The bottom section has a pull out portion that exposes the access hatch. The upper section rolls back easily out of the way without needing to remove the whole thing.
Each of the mattresses are 4″ thick so the overall thickness of 8″ is very comfortable. A side benefit is that the top mattress is much easier to make up because it is quite light and only 4″ thick.