Mobile home exterior doors are made like a sandwich with a metal or vinyl sheet on the outside, a Styrofoam core and a metal or vinyl sheet on the inside. They are manufactured to meet the minimum HUD codes and no more. That means they will seal tightly when closed and keep the weather out….
The first time I saw the multi-inch gap at the bottom of a mobile home interior door I thought some really bad carpenter had been at work on it. Of course, I learned later the large gap at the bottom of every interior door is deliberate and provides the cold air return to the furnace….