The walls are under 2" thick and are sandwiched to the aluminum studs. (Luan to stud, Styrofoam, interior luan to stud) a molly bolt is tough because the interior of the wall is usually Styrofoam so a molly will not expand on an exterior wall. The easiest way to find a stud is to look outside in the morning when your heater is on, you will see the studs.
The best way is to use a drywall anchor behind your accessories. If you hit a stud, switch to a self tapping screw.
You can measure your door jamb to get wall thickness and use a screw shorter than that thickness.
Ps. The interior walls are typically 2x2's and are hollow.