Hint if you want to build a 3! CAD model of the inside of your van start by establishing a fixed point inside the van as your temporaty 0,0,0 location until you establish a preferred one . Everything is located related to a 0,0,0 X,Y, Z. A factory installed hole on the floor in the rear of a cargo van is a good temporary 0,0,0 to choose as you can set up some large, accurate squares and additionally use laser levels to get vertical and horizontal reference lines, If you wish, for convenience, the roof beams can be marked with measurements for a quick reference as can be the floor and the ribs on the wall. But do double check to make sure those marks are accurate related to 0,0,0 in your CAD model.