RTAB-Map stores its map information in a database (by default, \$HOME/.ros/rtabmap.db). I tried out the demos, and as far as I can tell it doesn't produce a 2D map that you "download". When the system is running, it makes a 2D map that it has built from its own data and publishes it on a topic. You could grab the map when it's published and save it to file, I suppose.