Solid wire (Romex) is commonly used in houses. Solid wire cannot be used in a moving vehicle. The constant vibration would break the conductor over time.
All wires used in your van build should be double insulated, or routed through split loom tubing, and secured with cable ties.
If placing a wire through the metal of the van, use a rubber grommet to protect it from the sharp metal edges. Also if you drill a how in the van body be sure to paint the raw metal before applying a grommet.