How about leaving the last foot across the back (where the curves are) flat, and hinging up the whole rectangle forward of that? If the mattress is not cut too snug to the side walls, and it is not too heavy, it should lift OK, even though only 2/3 to 3/4 of the platform hinges up.
This idea reminds me of my bunk lockers aboard US Navy ships.
I did this on the Burro
. Used 3/4 plywood. Rear curve area fixed to Burro
bench seats about 9 inches so the storage area is still useful, front bed area hinged to fixed curve board and goes across both side seats.I used cardboard to make the curve templates. Insulated the seat opening areas later as it got cold in there and transferred through. The real mattress that came with the trailer doesn't seem to mind that the whole base is not lifting, it just slides forward a little as it's raised.
Hinges up beautifully, I was looking for gas struts but the science involved in choosing the right one was too much. Check the WEB, very confusing. Too little pressure and the bed is too heavy, too much and it comes up easy but you can't get it back down.
So I used a couple of 2x4's on each side to prop it up.
Going across the seats completely leaves a gap ( on the Burro) where the table would have put in when in bed mode. I made a piece of plywood to fit this area and screwed two legs (2x4) to the front of it so it would slide out ( like the old gaucho beds in some older and a few new campers. We put a piece of foam in this and can now sleep with or heads at the rear window and feet going toward hitch. Much more comfortable
But I digress, the only problem with lifting the mattress is it tends to rip out the window screening when lifted up ( rubs hard against it ) the Burro
goes narrower at the window point. If your trailer doesn't narrow, it should work just fine.
I may go to hatches anyway.