According to my known, almost all commercial RTOSes have the simulator which can run on Windows and/or Linux PCs. For example, Wind River supply the vxsim that can run on Windows and Linux. MG supply the Nucleus Sim. And ...
In Vxworks there is VxSim which can inturn simulate ur hardware independant software modules from the Shell Script while the Host will itself behave like the target .But the modules should be accessible with global variables ...
So , it is not true that u cant simulate ur software in RTOS ... but it is true that u cant check it if it is fully hardware dependant .
Just find the proper IDE for the processor type you want to use - most modern IDEs (KEIL, IAR etc) have built in simulators for the processors they support - and give your RTOS a try.