I would just take off the bunk cushions and put a solid support, like 1/2" plywood all the way across. Get a memory foam mattress close to the right size and cut it to fit. You could upholster the backs of the dinette seats for a headboard and footboard if you wanted to.
I would also get a drawer put under the dinette seat on the side not blocked by the kitchen. And you would have the space in the middle for tubs, large items, etc. You would lose easy access to the space under the dinette seat that is blocked by the kitchen.
If you use a memory foam mattress, you need the support to have holes in it for air flow, or use some stuff called HyVent under the mattress. Otherwise you body moisture will condense under the mattress and can cause mold.