The magnetic switch is a small glass tube with two reeds inside it. it's burried in the door jam, likely near the bottom on the latch side, and it's the screen (inner door) jamb at that. You can not see it so don't bother looking.
You might be able to find the magnet, it's in the bottom corner (Latch side) of the screen door on most RV's.
IF the screen door is bent so that the bottom corner does not come in all the way..... that will do it, (cause your problem) Normally this can be fixed by simply 'Adjusting' the door (Take two hands, apply firmly and adjust (Bend) the door).