If it is the typical Velvac mirror that is in use on most Class Cs.
You can remove the mirror head from the arm, and as you can see in the photo there is plenty room around the shaft on both sides to put a screwdriver or something else down beside it and fish out anything that is in there.
I would think that is how they got in there in the first place.
2006 TIOGA 26Q CHEVY 6.0 WORKHORSE VORTECFormer El Monte RV RentalRetired Teamster Local 692Buying A Rental Class C