When you speak of ventilation, are you talking about between the insulation and the outer skin of the box?
Can You explain this with a pic?
and I was planning on notching a hole at the ends of each joist and alternating the holes, then having a fan that can be snapped on, or run on a timer.
This is not a good idea:
to coat the original wood floor of the truck with fiberglass resin to help prevent moisture damage to the floor decking. Am I on the right path in my thinking?
In structures, water flows down, but also from warm to cool, thick to thin. Plywood is problematic because it is layered thin-thick-thin-thick, thick glue between layers of wood. When creating a temperature difference of at least 2°C the water starts to flow towards the cold. Steam pressure is higher in warm than cold. In case of barriers, it will be stuck and stay for prolonged periods - to reach temperature or pressure difference to flow again. For this reason, flooded houses are tough to dry. Standing moist feeds all kinds of fungi and other micro-organisms and they tend to cause problems. Not to mention the freeze-thaw cycle in a colder climate. Water expands when it freezes causing moist structures to break.
For example, freight containers or vans have this problem - the density is reversed, the thick side is outermost layer and water has nowhere to go. Therefore it condensates and drips down. The worst possible mistake is to use mineral- or other kinds of wool insulation inside. I see this a lot.
Spray Foam PU attaches seamlessly to metal surface leaving no space for water. It also mutes the temperature difference from the surface, so the moist (steam) stays in the air and then that needs to be vented out.