Mocked up the interior of the van with duct tape and styrofoam. Thanks to Sportsmobile Forum users for advising this method. We learned a lot through the process, and then redesigned several aspects of the build.
We used styrofoam to build some pieces of furniture that we needed to see in 3 dimensions. We used tape to mark outlines for the rest.
That’s A LOT of work, but it’s an invaluable exercise. Pre CAD-CAM tech, they used to design everything that way.