I'd not carry a gas can in the trailer, if I can help it. I'd consider a RotoPax mount and have the cans mounted on the exterior somewhere.
As for a roof vent, make sure your battery on the trailer can handle it. I'd personally go for a Fantastic Vent model that has an Ultra Breeze vent cover, so the fan can be used in the rain. I'd also consider adding a vent or two in the trailer to create an airflow for incoming air... perhaps with a filter in place so the rig doesn't get dusty.
The Fantastic Fan uses about 2 amps on high, so you will need to find a way to replenish the 48 amp-hours per day used. At minimum, if you can get a 200-250 watt solar panel and a cheapie PWM controller, that would offset the battery drain.