Annoyingly, Crestline don't publish a specific maximum tyre width figure anywhere obvious, but the search results do give us some useful real-world data points to work with. Here's what I can say with confidence.
The RS 181.2 is described as having "generous tire clearance" on Fanatik's product page, which tells us precisely nothing quantitative but at least confirms Crestline are aware it's a selling point.
In terms of what owners actually run: Pinkbike's review bike was running Schwalbe Magic Mary front and rear, and from our community data,
@Rob Rides EMTB has noted the BB height measurements were taken with a Schwalbe Albert 2.5 rear and Magic Mary 2.5 front
- so 2.5" clears fine on both ends.
@John_likes_bikes ran a build with Continental Kryptotal 2.4 Trail rear on his RS181.1, and the Spectre build data we have shows Maxxis Assegai 2.5 front and Minion DHR II 2.4 rear without issue.
What Crestline
don't specify is a hard upper limit. The practical ceiling the community has pushed to is 2.5" on both wheels, and given the tighter clearance around the linkage area (worth noting: the S 180 had rear tyre clearance issues in heavy mud with a 2.5" Albert, which packed the frame sufficiently to stop the wheel spinning