Don, I agree with you, and that's certainly what I've been doing ... letting the EGC of the feeder provide the connection to the grounding network of the main building.
Yet, this very forum, quite recently, said differently. I believe it was Bob (Iwire) who led the charge - that the NEC prohibits the use of the EGC as part of the GEC connections. That leaves us with the necessity of running another ditch, just to connect the electrodes.
That's why I connected these different assertions to this example of the mobile home .... to highlight the absurdity of the concept. I can hardly wait for the HI forums to discover this "error" in every home with a detached garage, every pedestal installation, etc.
On the detached garage situation I think it is pretty clear. The separate building or structure must have its own grounding electrode system, and does not need to run all the way to the building or structure that contains the service, it just needs to run to the main disconnect for the separate building and be connected to the equipment grounding conductor at that point. The issue here is whether or not the pole/pedestal and the mobile home are separate structures or not, as far as the NEC is concerned for purposes of supplying power to a mobile home.
If you did have to bring the grounding electrode conductor from the home to the disconnect why would it need to be in a separate ditch? Why would it or the feeder need to be underground? I do agree that underground is the best way to do it most of the time but do not feel it has to be underground either.