The best way to understand what should go in your camper is to put it in your camper, or at least a camper-sized space.

If your build is sufficiently far along, actually put each item in your camper when you decide to keep it. The amount of available space and need for organization will become apparent.

If you don't have a camper (or have the vehicle but can't load it up yet), measure the camper's floor plan and mark it out with painters tape on the floor. Literally place each item you will bring in the space as if it were your camper.