While RealPage might command 80% of the market for this type of software, they only have 12,000 clients. There are over 5.2 million multi-family dwellings in the US. It's only a monopoly in that they offer a very niche product. So I doubt the implication the justice department is making here - that RealPage is having a significant impact on market price through widespread collusion.
Furthermore, housing is a market - nobody is "competing on merits". There's limited inventory, and it goes first-come, first-serve. Realpage advertising that it gets the best dollar for its clients isn't that much different than your Schwab account letting you know you shouldn't sell your MSFT share for $50. I suspect the DOJ may have trouble actually proving that landlords held to the price recommendations to their own detriment to keep them high.
While I appreciate the breaking up of a potential cartel here, and this is a software I would never use, I would hold my breath if I was expecting a sudden change in the rental market because of this. Inventory is still fundamentally limited, and unless the DOJ bans all market research, the going rate is still the going rate.