You didn't say what van. Ford or Sprinter. I had a 2005 Pleasure-way Sprinter Legend. I am assuming you are talking about the ottoman and infill cushion part. The Pleasure-way has two positions for the ottoman seat cushions. You have to carefully know how to position the seat cushions and when you get it right the two infill back cushions then fit tight between. I originally just thought it was sloppy construction but after communicating with others I found it to just be the case.
I also installed a removable slat to prevent the cushions from sliding out. I did this before I figured out how to seat the cushions correctly.