I think it depends largely on experience. Myself, I have very very little experience - this will be our 'learning' coach. That said, even printing off the PDI check list, I felt more comfortable hiring someone to go through it and watch and ask questions as we went.
he was AWESOME - I can't even imagine having done the inspection without him. He knew a lot, and was able to help teach me some new things. Well worth the money, but as inspectors go, he was VERY inexpensive - around $100 - vs the 600 to 900 i saw advertised on line.
Once we are out in the 'field' so to speak and learn more and more, when we decide to change coaches in 5 or so years, I'd feel more comfortable doing it on my own. well, as long as I am physically able. I'm not able to crawl on the roof or under the coach now, but hope to improve that situation. all in all, I don't regret hiring the 3rd party (not someone at the Dealership).
Good luck with whatever you decide!