I've come across with exactly the same problem a couple of hours ago, and, strangely, removing and re-checking out the p2os from the repository solved the problem as it is explained here.
Besides, be careful about the /use_sim_time parameter. If you run gazebo launcher(pioneer3dx.gazebo.launch) first, -since it sets this parameter to "true" via empty_world.launch- launching the pioneer3dx_urdf.launch after will not yield a proper visualization. Either set this parameter to false by "rosparam set" or add
<param name="/use_sim_time" value="false" />
to your custom launch file or to top of the "pioneer3dx_urdf.launch".
Lastly, after launching "pioneer3dx.gazebo.launch" file, the links connected to base_swivel_joint and swivel_hubcap_joint cannot be visualized properly in rviz since these joints' states aren't published by gazebo, and robot_state_publisher cannot publish corresponding tf messages. Yet, I'm not sure how and where this can be added for the moment.
