Quite simply, it is true. Looking at the most recent data here at UWM, students in University Housing achieved “Satisfactory Performance” at a rate of 79.1%, while commuters were at a rate of 65.1%. In terms of GPA impact, a study performed at a state school in Indiana appeared in the Economics Bulletin^ and found that the effect was between .19 and .97 GPA points.
Pedro de Araujo and James Murray, (2010) “Estimating the effects of dormitory living on student performance”, Economics Bulletin, Vol. 30 no. 1 pp. 866-878.