Two Mobile Home Questions (from Non-Mobile Home Owner)

1) If a home is on a foundation, is there a way to access floor-based forced-heating/cooling? I'm amazed by the feces, cigarette butts, mold, dust, cooking grease, pet hair I find when I visit these senior citizens and shut-ins with respiratory problems, but none seem to ever consider that their home is making them ill. Yesterday, I counted an eight-vent floor in an extremely tiny home. If cleaning this style of duct is impossible--I don't mean blowing them with a Shop Vac, I mean cleaning the mold out--is there a way to calculate total tear-out and replacement?
2) Why do mobile home owner/landlords restain woodwork or panelling so often? I thought it might be some kind of anti-termite measure, but twice in the dead of winter (Pennsylvania), I've been in newly- restained homes that made me ill. It was like breathing pure turpentine. Termites can't be that active in sub-zero temperatures, right?
Thanks.
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I'm not an expert but my mother lived in a older single wide for a few years in FL. The vents were sheet metal between the floor joists. Then sealed with from the underside. Plumbing was the same. In FL they slowly settle into the sand so there was not really room to get under it anyway. We pulled up floor from inside to get at a drain problem. I don't know about newer ones but it struck me that maintenance was not given much consideration in the older ones. They lasted about 30 years and then seemed to be disposable.
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After I posted, an old(er) lady told me formaldehyde was used heavily in mobile home construction. Now I'm wondering if that's the reason for all these wall tear-outs and replacement with cheap(?) plywood, and that awful stinky stain. I mean, what are the chances of going to three mobile homes in three different parts of one county and finding all-wood walls (pretty nice to look at, actually), but all with that awful stink?
Thanks for the head's up about the floor issue, which has to explain why the heating system can never be cleaned.
Sad. Your mom was lucky to have you.
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