I don’t know for sure but it is probably a queen or an RV queen which is just shorter. If you get a memory foam queen and the berth is shorter you can easily cut it down to size with an electric kitchen knife.
The bed is between a double and queen by about four inches. Measure between the “end tables” to obtain maximum width of a potential mattress. Pretty sure a standard queen is too wide for that space. We use a thick and dense memory foam cut to size. Works well for us.