Don't know if this will work for you but they have some very good ideas:

https://www.parkedinparadise.com/campervan-bed-design/

Our ProMaster Camper Van Conversion – Interior Layout

https://www.buildagreenrv.com/our-conversion/our-promaster-camper-van-conversion-interior-layout/

https://www.youtube.com/watch?v=9LXhC6FxQOk

Perhaps this will give you some ideas. https://www.hometalk.com/43883849/organize-your-camper

I looked up the dimensions for mattresses and found even a twin mattress is wider than 28". Had you thought about getting a piece of thick upholstery foam cut to your dimensions? You could put it in a washable zippered cover. What type of base were you thinking? You wouldn't want too much added weight. Good luck and stay safe!

P.S. here's a link for you:

https://www.parkedinparadise.com/campervan-bed-design/

Hello Robin,

Use old Pallets. Cut ro suit as a frame. Screw or nail together. Overlay with MDF or Plywood then , Add a mattress or slab of fire retardant foam. Sweet dreams.

here are more ideas

https://www.youtube.com/watch?v=cpC7NpRWiy4

https://www.youtube.com/watch?v=VbEt_rAour0

Here are some directions from Pinterest https://www.pinterest.com/pin/213498838574198133

Do you want it to hinge up out of the way? A 2x6 frame and plywood base should give you about what you want.

something like this? https://www.charlestoncrafted.com/how-to-build-a-platform-bed-for-50/

Why not just put a mattress on the floor in the van

I built one for a friend using plywood top with 2x4 frame and legs. Built it high enough for him to slide plastic baskets under it for storage. He really likes it.