Both of these houses are in good condition and have long term tenets living in them. (6yrs) and (3yrs).

You need to look at the sale of duplexes close by to start formulating what they go for, then perhaps add a price adjustment upwards for price/condition, assuming other duplexes are old.

A family began to rent our newly built house in June and after about a month, the wife had been diagnosed with a serious medical condition.

Similar square footage and condition.

These are properties that have sold near the subject property with a similar: build/construction type, lot size, building size, interior condition, property type, year built.

Houses in similar condition etc as yours.

If the firm you approach does not offer these privileges, keep searching until you find a company that will allow you to contact the higher management whenever you want to discuss the condition of your property with them.6.

In addition, a person cannot discriminate in the terms or conditions of a rental agreement against another person based on source of income, or based upon the person's participation in a 3rd-party contract required as a condition of receiving public housing assistance.

I have researched a sample "Notice to vacate due to non livable conditions", but have not found anything. 

Also, if possible check on the age, size and condition of the type of mobile homes that can be put on the land.