I live in a “double-wide” mobile home that was built about 1960. We moved into it in June 1987. It has one register for the heating system, which is the very tail end of a heating duct that extends out to the west wall (the dining area) at one end then doubles back, and the floor register for my room is near the other end of the place. So many parts of the “double” half of the dwelling seem to be afterthoughts, such as the electrical connections (outlets, porch light on the side, etc.).
But what disturbs me is the lack of heat; and a carpenter who has done some remodeling for us, including replacing a louvered window in my room (I could never get it properly closed), said the room suffers from mildew. Near as I can tell the floor in my room is the same as elsewhere, but it may just be that the floor is letting moisture as well as cold in. (It’ll be cold in my room–and warmer outside the mobile home!)
Any reason for this? 
I used the SDMB Search engine to get answers about mildew and found seven pages of thread. But to solve a mildew problem in my mobile home here in Los Angeles County, I’m batting zero.
Any answers, please?
Ah mold - the bane of mobile homes everywhere.
Mold like to grow where it’s warm and wet. And in a mobile home, walls and floors heat up very easily. Moisture is trapped inside from normal living (cooking, bathing, breathing) and has trouble getting out. Moisture then sticks to the walls and floors, which heat fast, and you get mold.
You might be getting moisture in from the floor. I don’t know about California, but here in Alabama, the pressboard floors in mobile homes will wick moisture into the home from the humidity of the outside air. You have to properly insulate and seal the floors to keep this from happening.
This EPA guide should help you.
Thanks, DeVena. 
I’ll log onto that site later when I have time.
Dougie_monty