No additional flooring on top, maybe some rugs near the front. I have a similar seam near the front but not as wide, I don't think I would be able to get any caulk into that one. My batteries are going up against the wheel well, and the rest of my gear will be stacked in tough boxes under my bed.
I am currently stenciling the floor and will use a top coat sealer to further protect it. The plywood is on poly foam board for insulation so there will always be some flex in the floor.
I think I found something that will work. I'll use clear silicone adhesive for extra protection.
Thermal and moisture expansion/contraction of the plywood will always want to pull the joints apart. Best result would be to glue together 2 thinner layers of plywood using offset joints. Two layers of underlayment grade plywood such as IntegraPly (5.5mm 5-ply hardwood) will make a very flat, stable floor. Use exterior grade carpenter glue (cheap, easy to spread, and not too thick) and "clamp" by covering the floor with heavy bags for 24 hours. Could be sand, ready mix cement, water softener salt - whatever you might have a use for afterwards. Or, just return the bags.