If you have the four cushions you have most of what you need to figure out what it would look like and how to build it.
The table would the same size as the two back cushions - because that becomes the mattress, that will give you width and depth of the table top.
The two seats are as far apart from each other as the table is wide - because the table top has to 'become one' with the seats, it probably sits on something that juts from the front of the seats - so make the board for the seat cushion about 1" wider that it needs to be to just hold the cushion.
The seat depth (knee to spine) is as deep as the cushions, and as wide as the cushions.
The height of the seat is whatever is comfortable for you.
The height of the table is whatever is comfortable for you based on the seat height.
Put some storage and the seats and you're on your way.
2013 Winnebago Sightseer 36V