I built a sliding slat bed for our family E350 Camping van. I found some photos online in Pinterest & Google photos and came up with a design that slides over the bench seat to make a pullout bed.
The basic design is to use wood slats that interlace like your two hands coming coming together. I built two separate frames. One frame is stationary and the second frame slides through the first frame.
This is how the bed looks like in the extended mode.
Sliding slat bed in compact mode.
This is the bed extended in our van.
Here is the bed in travel mode.
Here is the basic design concept.
I did not include dimensions as each van is different. I bought stock lumber and cut to length. The slats are 1" x 3". The frames are made from 2" x 2"s.