Federal Motor Vehicle Safety Standards have the mirrors put on when the chassis is manufactured (it starts out as a van). Thus even U-Hauls will have the mirrors, although there is nothing there to see.
Whether or not the mirror is useful in a Class C RV depends on the design of the house. I can see what's going on with my passengers, and back to the kitchen, and what doors might be swinging. If it is all clear through the house, I can see my towed vehicle in the wide-angle lens on my rear window.
If the mirror gets in your way, it can be slipped off the mounting puck, and the puck removed using acetone (maybe nail polish remover).
