I expect the gotcha here to be the access that RealPage has to non-public data. It's basically the same as two landlords not saying they'll collude to raise prices but putting all of their private rental data on the other's doorstep. Makes it really easy to figure out what the other's going to do if you can see all of the information they use to make decisions on and, thus, what you can get away with.
If RealPage only aggregated data they scraped from public sources and then applied an algorithm to find patterns within that data then I don't think there would be much to this lawsuit. As unfair and one-sided as something like that would be I don't think it would fall afoul of any collusion laws.