See
http://wiki.ros.org/rtabmap_ros#Map_Cloud_DisplayIn rviz, are you visualizing /rtabmap/mapData topic? If so, the max depth range is in the options of the rviz display as in the link above.
If you are visualizing cloud_map, it is indeed Grid/RangeMax that should be modified. Note sure how you launched rtabmap node, but Grid/RangeMax can be set as parameter inside rtabmap tag, or by argument (see
doc).
When doing "rtabmap --params", it only lists default parameters with default values.