I inspected a pre-manufactured house from the 70’s. The new owners wanted to know how to improve comfort and air quality in the living space. I crawled under the house and found what thew image shows. The typical insulation held up by mesh fabric was missing and 2" XPS foam board was attached to the bottom of the frame but with large gaps rendering the insulation useless. The skirting was OSB with direct soil contact, the roof had no gutters which directed all rain and snowmelt from the roof to the skirting and to the soil under the house. There were sections of skirting that had rotted away. So, essentially, the house was sitting on top of the exterior environment.
that had no venting
Location, location, location?
Depending on where you are, no insulation is typical. Although you said snow, so probably not where ever you are.
That house has some serious moisture and possible mold problems. I would recommend they hire a qualified Industrial Hygenist to do some mold sampling. It looks like a big remediation job if it turns out to be toxic mold.