[rascul]

I see many of them that simply vent into the attic. This requires a properly vented attic, and has the downside of introducing moisture into the attic, which can lead to damage.

That said, mobile homes pretty much always vent outside in my experience.

[stefan_]

A lot of them do not vent at all. They just draw it up through a coarse filter and out the top again.

[rascul]

Yes, I see those too. I work on houses for a living though and those are not too common, at least around here. I typically might see those in the older houses, or in half assed renovations.