I've done the total opposite to my dinette but am with ya on what you're trying to accomplish!
Is your dinette currently 2 benches with a table or a U shape dinette with a small table and storage under the window as well?
Do the benches currently have drawers already built into them or will you need to add those as well?
The reason I ask is because you might have to do some exploratory demolition and remove material before you start designing and building what you want.
My initial thoughts would be, leave the two benches (If there are not drawers, use the existing cabinetry and 36" drawer slides and build boxes) and remove anything in the middle. Then, build a face piece and use 36" drawers slides to build a large 36x36 drawer. Put plywood over the opening and trim it out as a nice bed.
Again, I did opposite. I took out the center of the U and left 2 benches making for more leg room and a larger table.
Below is a link to my thread. PM me if you have any question or want to bounce some ideas off me.
U Shaped Dinette Begone!!!
2017 Forest River Surveyor 247BHDS...
...Tugged by a 2016 F250 XLT FX4 SuperCrew, Shortbed 6.2L, 4wd, 3.73...
...Joined with a Husky Centerline TS 800-1200lbs WDH