I understand the cartel allegations here, but I think people are vastly underselling the competitive forces at play. If you are not filling your unit immediately, you are losing thousands of dollars a month.
Cartels break down because of the incentive to undercut (prisoner's dilemma). But in this case, it would be very, very profitable to undercut RealPage's prices and get your units filled before them. So their compliance and enforcement mechanisms of RealPage would have to be extremely robust to get corporations to willingly lose tens or hundreds of thousands of dollars a month to collectively collude on prices.