We've bought both new and used and had problems of one sort or another with both. We bought our Monty used last spring after personal inspection even though the dealership claimed to 'completely inspect and test' everything. We've had it out four times so far, one a 2k + mile trip, and haven't had anything big pop up, although there were a few minor things (mostly from the manufacturer).
Our former TT and a Class C I bought years ago were both new purchases. The Class C had three big issues before I even took it camping the first time (one was the fresh water line from the tank to the coach wasn't connected right & it leaked all over everything!). The TT had more minor things than our Monty, but nothing what I'd consider major.
I believe that anyone purchasing something this expensive would be asking for trouble if they didn't conduct a thorough inspection of everything. This includes making certain everything works - even though a dealer might not want to put water in it or have you watching the slides move in & out, etc., before signing on the bottom line. Too many folks fall in love with what it looks like with the slides out and never do anything else. The one that really gets me is how some newer 5er models make you stack recliners or do something equally unrealistic to just pull the slides in. An older buyer of one of these posted a message on another board asking how he could avoid all that. Why he didn't find that out beforehand is beyond me.