It is probably fair to say that few first time buyers, if any, can actually do a good PDI, even with a checklist in hand. More seasoned buyers are in a better position.
I don't really put much stock in a dealers representations. It may be true that buyers get cold feet, but it is also true that from time to time buyers end up with a unit with major problems and those buyers often say they pretty much relied on the dealer's prep and PDI process, and took it on good faith.
So somewhere in between is helpful, regardless.
It really is about risk minimization as to the potential for warranty problems and time spent with the rig in the shop. Of course, a trailer is different that a motorhome, so there is that as well.
Just as a note, when I do a warranty inspection on a unit, it is preferred to have it fully hooked up, as what is not hooked up can't be tested.
Note: the plumbing post is a good example of a partial PDI and the unfortunate result. On the other hand, diy owners tend to take their tools with them and do most of their own warranty work. If I need a part, just send it to me. Despite being an old fart, I can still do just about any of the work myself unless it needs a service bay or two people.