MY floor won't drop out

I've nearly finished prepping the floor of the van. It has been de-rusted and given a coat of primer. I just need to finish sealing all the joints and seams with fiberglass, then one more overall coat of primer. Sealing all the seams may be overkill, but this van is my future house. I'd rather overkill than wish I'd done better, later. Do it right, do it once. The next step is to install a plywood sub floor, which will level out the current uneven state of the floor, and serve as a secure attachment point for my interior structures.
I have also taken painstaking measurements, so I can draw the van to scale and come up with a "floor plan" for the bed/kitchen/cabinets/battery box, etc. I will also determine where I need wiring, so I can install it before I put up the insulation and "walls". I've always liked scale drawing. I've used it in innumerable carpentry projects, and did a stint as a landscape designer for a while. Drawing and writing are great tools in my creative process. They help me generate ideas, see what will work and what won't, and think through the constuction process so I don't leave out critical steps.
Measure twice, cut once.