Do you know what's underneath? 

If the sheet rock is soft and the screw driver easily goes into the wall, you have to remove a 1 foot by 1 foot section to see if there is mold.If you have linoleum or vynal floors, look for corners that have peeled up and see if water is getting into there.If the house is on pilars or blocks crawl under the house to see if the soil is damp and if the wood is rotting.

Houses need to be maintained, fixed up, rehabbed (like his does)  and that it takes time & money to keep a property from becoming potting soil. ;-) The "useful life" theory.  

In the past 2 weeks alone my good neighbors and I have witnessed hand-to-hand drug transactions right on the curb while looking out the window in the middle of the day, prowling creepy trespassers hiding out underneath our decks and on our porches in the middle of the night, a huge amount of vehicle traffic visiting the property all night and even a parked car on the street with two adults in it coming down from very hard drugs (authorities were called promptly). 

Removing dirt can cause a foundation to shift and affect houses around you so either way, you'll probably need a soils and hydrology test. 

Manufactured will most likely have steel beams underneath and be designed to work with or without a permanent foundation.

Also, soft soil areas tend to shift as opposed to resting on bedrock which is more stable.

If your market has older homes, known bad soil conditions, weather events, etc etc.

In my area this situation is common was mostly caused by work done 75+ years ago before modern regulations existed.In fact I own such as house where the house behind me has their sewer connecting to my line underneath my house!

And, you want to make sure the soil is 6 inches below the exterior concrete of the foundation.