The difference is that the information gas stations are using is public, but the information used by RealPage is non-public and sensitive.

From https://www.ftc.gov/advice-guidance/competition-guidance/guide-antitrust-laws/dealings-competitors/price-fixing

If there is evidence that the gasoline station operators talked to each other about increasing prices and agreed on a common pricing plan, however, that may be an antitrust violation.

…

Each company is free to set its own prices, and it may charge the same price as its competitors as long as the decision was not based on any agreement or coordination with a competitor.

So the question becomes: does use of RealPage qualify as “coordination with a competitor?”

From https://en.wikipedia.org/wiki/Price_fixing

Under American law, exchanging prices among competitors can also violate the antitrust laws. That includes exchanging prices with the intent to fix prices or the exchange affecting the prices individual competitors set. Proof that competitors have shared prices can be used as part of the evidence of an illegal price fixing agreement.5 - source Experts generally advise that competitors avoid even the appearance of agreeing on price.[5]

In this case you have them sharing prices with RealPage in exchange for information that will affect their prices.

I’m no lawyer, but it seems pretty clear that this situation could qualify as price fixing.