School me on heated tanks and enclosed underbelly stuff

Thanks for all the replies. I'm wondering on the ducted heat versions, is there an exit. So it goes down in there and yet eventually comes back up into the coach area? Some of you were complaining about your bedrooms being too cold because the heat is used up down there! How can you tell just by looking at something if you're buying it off of Craigslist if that is how it works is there a way to figure that out?