The oxygen tension is already lower yet you're wearing a mask which will impair your ability to clear your eustachian tubes, circulate and humidify the air in your nasal passages.

There is nothing growing on the walls or anything, but if you go in there and sit on the couch or the bed it feels cold and damp like it was in a refrigerator or something.I don't believe there are any leaks or anything, I pay for the water bill and the bill is very reasonable, ~100 bucks a month for all three units.There is a crawlspace on the other side of the wall of her unit where the water heaters for the units reside and there are various things stored like yard tools, is it possible that rain water is getting into there somehow and leaking under the floor, humidifying the room?

I run 1 Whirlpool humidifier as needed snd it does the job (best value I got online from Costco).

Tried using a humidifier in the home?

Patrick,  A humidifier would be good for after you close to help get the moisture under control, so would be just letting the house breathe with the windows open.

I have spoke to an inspector / mold cleanup business, and they have gone through it, and feel that at the point it is in right now - it is too cold for mold to continue to grow, and they can lick it right now fairly easy, then the drain field is done, and a de-humidifier for a few weeks, and it is good...

I remove whole house humidifiers and water softeners.

Replace or repair fixtures as needed.The low-flow items can help if your tenants leave the water running during various activities - tooth brushing, face washing, kitchen activities, etc.Depending on the heating systems, be sure to check that humidifiers aren't wasting water.

My husband used to go down to the basement and clear out any dirt, check the humidifier and make sure the basement was well ventilated. 

Always running bathroom exhaust fan during showers, do not use humidifiers, use air conditioning when it is humid outside, report any water leaks to landlord immediately, report any mold immediately.Holding them liable for mold damage without regard for what caused it is pretty hard to enforce.