Measure the distance from the bottom of your camper van door to the ground when the camper is parked on a flat surface, to get an idea of exactly how deep to build your boxes.
Measure the location where your boxes/steps will be stowed while in transit. Choose a spot where two boxes of varying sizes can fit snugly and are easily accessible. Ideally, your larger box will be around 12 inches deep, 18 inches wide and 12 inches long, with your smaller one being a few inches smaller than that.
Build two boxes of the same width and length, but varying depths, using a drill, screws and 3/4-inch or thicker plywood. If you do not have a saw to cut the sides to the proper size, have them cut at your local hardware store. Ideally, your taller box's depth should be 6 to 7 inches shorter than the length of distance from your camper van door to the ground. The next box should be another 6 to 7 inches down. The standard "riser height" for stairs is usually around 7 inches maximum (see Reference 1), so do not make the smaller box less than 7 inches shorter than the larger one.
Place the two boxes upside down so the open hole faces down. Place the boxes next to each other so their wide sections are lined up, and they create a higher and lower "stair." Install slide bolt hardware on both sides of the boxes to join them together while in use as a set of stairs. When you need to stash the boxes, undo the slide bolts and use the two boxes to store camping goods.
Install self-stick utility carpeting or matting to the bottom of the boxes to make them soft and slip-proof when they are in use as stairs. When in use as storage boxes, they will keep the plywood from scratching your camper van interior.
