I usually start the process by modeling the robot using Simscape and then exporting the rigid body as URDF. Simscape allows you to build very complex rigid body assemblies and simulate them. You can then hook up your model to controllers defined in Simulink and have a complete simulation setup. Give it a try.
Here is a link to how to build a robot using Simscape:
https://www.mathworks.com/matlabcentral/fileexchange/119268-excavator-design-with-simscape