The other is they now want a vapor barrier installed in the floor but these are older model trailers “as is” and we haven’t had an issue with mold or moisture.

Stop getting the vapors and answer the questions.

Anyway, you MIGHT (or might not) be able to get away with a vapor mitigation system to treat the indoor air issue.

if there is residual contamination below the slab, it could pose a risk of vapor intrusion that could prevent you from renting the property for residential.

The geologist and toxicologist hired by the plant for ground and air sampling has assured me that the property is not in the contaminated area (I have these contacts because my current residence a block over has vapors coming in through the foundation and remediation in progress). 

What you really need to address in those pictures is the vapor barrier that's pulled away from the dirt.

Fresh paint, floors/carpet, plumbing repairs, electrical repairs, soffit repair, vapor barrier in crawl space, all sorts of stuff!

That's actually a great opportunity to add an air/water/vapor control layer, too.One option for the heating would be to add mini-splits to each apartment.

Needs a vapor barrier as well. 

Perhaps the most useful attachment for it you'll ever use will be organic vapor cartridges.