Welcome to the forum Talula:
Some mobile homes have screws in a flange around the outside of the window with a soft putty behind it. It takes a special figure 8 screwdriver, you will need some insulation (depending on the wall thickness), and some exterior wall covering (corrugated galvanized steel or roof metal will work just fine). You can get the roof metal in several colors and I wouldn't try to match colors, it won't work. You could make the window covers look like shutters or like an awning that is folded down; just make sure you use a flashing on the top that goes under the siding and comes out over the window treatment to drain the water away, also caulk around the cover. When you go inside then, there should be no daylight showing. You may wish to add some studs in place of the windows; maybe one in the center of each opening, the drywall shouldn't have to span over 16". Now, fill the wall with insulation (if it is 2" thick I would use styrofoam board, otherwise use fiberglass) and go to the drywall. This is windy but I'm sure you will have more questions later.