I wouldn't open the trailer floor and expose it to the elements.
Installing a vent to the living area is what I would do. If it's still to warm in the bin you could install a small desk fan on a plug in thermostat, provided your inverter has a receptacle.
2013 RAM 3500 Dually Longhorn
2014 Trilogy 3650RL, W/D, Onan 5500, 8000# Axles, IS, 960 Watt Solar, 6x6 Volt Battery Bank, Freedom SW 3012 Inv/Charger