All dry food stuff is sealed in packaging and plastic containers.

Every residence has a private yard and large patio terrace.

@John Collins A great option would be pavers(brick) patio.

If I make it a 1 bed, the light from the patio door at the back would flood the place and allow a nice entertaining area.Thoughts?

They complained so I immediately had it professionally treated and I resealed the patio door.

There are other options, like adding a small vent fan to vent the crawl space to the outdoors, putting a dehumidifier in the crawl space, and having the crawl space "sealed" - basically spreading out big sheets of plastic on the ground and running them up the inside walls.

I've noticed that some of the soil is starting to wash out from underneath my patio slab. 

Often this isn't a paper throwaway filter like a central A/C has, but a section of wire or plastic mesh in the front grille that you can pull out and wash. 

They have industrial fans running in multiple rooms including the bedroom, kitchen and hallway and they are sectioned off with floor to ceiling plastic.

Good luck, sounds like a win-win. he smokes cigs. is it ok to allow him to do so on the outdoor patio?