Storage in a van is as personal as the user makes it for their own needs and depending upon the space available inside of their vehicle.
Most Vandwellers have their bed put up onto some type of frame, which gives them much more internal storage space. Depending upon the layout and the mattress chosen, the space has many possibilities. If one mattress is used (such as a futon), generally it's easier to get beneath if it can be moved to one side or front into a couch, then the storage beneath it can be accessed. Sometimes long drawers are made to pull out behind the van/minivan. Two or more bed cushions can be used which will allow for even easier access to the floor storage.
Other forms of storage can be built in cabinets, plastic tubs, or re-purposed plastic file cabinets.
Small items can be put into hanging racks such as are made for shoes or use in bathrooms.
Many ideas are available through YouTube searches on van dwelling and on Pinterest.