Blind Spot
The area beside the vehicle and diagonally behind it that the driver can see neither in the mirrors nor by direct sight. As the vehicle gets longer and a trailer is added behind it, this area grows; when towing an RV the blind spot extends over a markedly wider area than in a car.
The blind spot is the gap between the field of view of the exterior mirror and the driver's over-the-shoulder line of sight. When the mirrors are adjusted too far inward this gap grows, because a significant part of the mirror then merely repeats the area already seen in the interior mirror.
When towing an RV or trailer, two additional effects come into play. First, the body of the trailer physically blocks part of the area the mirror sees. Second, since the trailer follows the inner path in turns, the area diagonally behind changes constantly. That is why looking only in the mirror before changing lanes is not enough; speed difference and distance must be judged in terms of time before the mirror.
The practical measures are these: adjusting the mirrors outward so that they show as little of your own body as possible, widening the view to the sides for towing, getting help from a spotter during manoeuvres, and doing a quick head-turn check before changing lanes. The mirror surface being free of mist and ice is a precondition for all of these checks.
The context in which this term is used: RV automatic levelling guide