Venting the cargo van for filtered air? As far as I know there is no filtered air unit such as a roof van available in the RV products market. You are going to be on the road in a air pollution generator with various reservoirs of many types of petroleum products... ie your van. You only have a very small cabin air filter in it for travel time and it is not going to be the only solution emitting vehicle on the road.
Air filtration 12v units for when you are parked? You can of course make your own filters for the roof fan units by suspending off the interior opening a filter typically used in household central heating and or cooling units.types. A filter will of course reduce the speed and amount of airflow from the fan. A real problem during a heat wave if you can't open all the windows and doors on a van.
As far as filtering out mold spores? If they are inside the walls coming out at you it will be a loosing battle to filter them out. If they are outside more airflow pulls them right into your van. Florida has tons of mold growing outside. Mold needs 2 things to survive, water and food. Dirt is a fantastic source of food, dirt on the ground, inside your house, on objects such as plants, buildings, sidewalks etc. Having spent many years in Seattle I am very familiar with common everyday mold that you see inside and outside.
regularly use a mix of vinegar and water to clean surfaces in whatever living space you choose. It has a residual effect and is a good mold preventor. The smell dissipates pretty quickly. Also use it when you do laundry, it keeps clothes free of mildew even after wash day.
