Hi,
You can use
pcl_ros/pointcloud_to_pcd node to save /rtabmap/cloud_map topic:
$ rosrun pcl_ros pointcloud_to_pcd input:=/rtabmap/cloud_map
To visualize the PCD file:
$ pcl_viewer cloud.pcd
You can also convert the PCD to PLY:
$ pcl_pcd2ply input.pcd output.ply
Note that you can always open the saved database (~/.ros/rtabmap.db) in RTAB-Map to regenerate the point cloud and export it using the GUI.
cheers