I think for that particular inspection, they do a pretty thorough inspection of the engine/chassis which you may or may not get elsewhere. I paid a mobile RV tech to inspect mine, and he mostly looked at the house side. He checked the oil, trans fluid, and brakes, but that was it as far as that side of it goes.
Diesels are expensive to fix so it may be worth it.
On the other hand, if I remember correctly, the guys from rvinspection.com DO NOT go up on the roof because of liability issues. Seems like a good way to miss potential water leaks. It COULD be another company that had that restriction, I'm not 100 percent sure on that, but I think it was these guys. Either way, it's something to consider when looking for an inspector.
If you do go that route, do as thorough of an inspection as you can on your own first. Hate to drop that much money to find out its a dud and then have to spend that much again to inspect another one.